It all really depends on the scene. Outdoors, indoors, tempature, etc... I been to some crime scenes where the incident happen a few hours prior and the blood is still wet and fresh looking. I been to other scenes with in a few minutes and the blood began to congel and dry up. You have to take all of the conditions into account.
Well as blood is exposed to oxegen it becomes red, while in the body it is almost a blue color. Once it begines to congeal it turns darker and darker red, almost a crimson color. Also you can see if it is wet as opposed to dry and almost a gel type substance. Imagine cutting yourself how it looks right at first and then several hours latter once it has become a scab on your arm. Also heat has a big impact on the appearance of blood. The hotter it is the faster it will coagulate and begine to break down. The colder it is the more preserved it is.
Actually there are white and red blood cells. Your veins have blood running through them. The blue color is due to the exposure of the blood to oxygen. This is the major difference between veins and arteries
Arterial blood is bright red - due to it being highly oxygenated.
Venous blood is darker red - less oxygen and carrying more carbon dioxide.
Veins look blue-green through the skin.
The only instance where this is not so is the veins that leave the heart en-route to the lungs (carrying low oxygenated blood to be reoxygenated by the lungs), and the arteries en route back to the heart (carrying reoxygenated blood back to the heart to then be pumped out around the body) , where the oxygen levels appear to be reversed from the arteries/veins of the rest of the body.
Maybe this is where the misunderstanding is coming from (all the answers above are correct more or less).
(Apologies if my diagram has made it more complicated - check on a medical or biology web site under 'deoxygenated blood' and 'pulmonary artery').
When arterial blood is left at a scene it will be bright red and slowly turn darker red over time - 'slowly' is a general term - the speed of changing is dependant on many factors, heat, damp, drying time due to air currents, whether is has foamed up, been spat out, splashed onto surfaces etc. That is why it is very difficult to date how long blood has been out of the body/deposited at a crime scene.
Venous blood is a darker red to start with, but will darken the same as arterial blood.
After a period of time blood will be a chocolate brown or even darker.
Dry stains can be bright red, darker red, or brown/chocolate brown, depentant on time, heat, air currents etc.
I would not expect a bright red stain, dry or wet, to remain bright red for long but I would not put a time to how long it would fade to red, or to brown.
I have heard experienced CSI officers and scene-going forensic scientists, say "that blood stain is old", when they were examining a 'fresh' scene and discover darker brown and chocolate brown stains amongst the fresher stains (for examiple at a crime scene where domestic abuse has occured over months or years).
I recently examined a scene where there was a red blood spot on a recently moved item (it used to be a vertical surface, but had fallen down during the offence).
I photoghraphically recorded and swabbed that stain as a 'more recent' and thus more likely to be an offender sample stain, than a chocolate brown smear on a fixed surface somewhere else in the property/house.
I could date the offence because it had flooded with water due to water pipes being damaged at the time. This had led to it being discovered quickly and workers were able to state that the pipes had not been damaged when they left the day before.
I could 'roughly' date the red blood stain to within 24 hours and infer is was deposited during the offence because it was on a fixture that had been moved by the offenders and had originally been a vertical surface (the blood had dropped onto it once it was laying flat on a floor in one of the rooms).
Forensic scientists, sometimes called technical forensic science or crime laboratory analysts, evidence used in criminal and legal fact-finding the truth of the matter at hand. The word "forensic" comes from a Latin word meaning "public" or "to the public." This name is fitting, because in addition to reviewing the evidence, a forensic scientist often has to give testimony in court about their findings.
A forensic scientist is often an area that he or she specializes in, including but not limited to, the areas of DNA analysis, firearms and toxicology. The field of wildlife forensics is considered increasingly important as the incidence of poaching increased worldwide. Similarly, forensic psychiatry is also in increasing demand as the justice system seeks to understand why offenders commit their crimes. Other forensic scientists specialize in dentistry, or the study of teeth, pathology, biology, or handwriting and document analysis. Each specialization requires continuing education and supplementary to the basic requirements to become a forensic scientist.
Forensic chemistry is a field of chemistry devoted to the chemical analysis of the substance or several substances that may be important or might have been used in the commission of a crime. A forensic chemist can evaluate substances that could be dangerous to others. For example, substances mailed Anthrax powder appears to be analyzed by a forensic chemist. Although in the past, chemists, forensic many had bachelor's degrees in chemistry in general and children in criminal studies, at present, many universities now offer degrees in forensic chemistry specific.
If you've seen the TV shows like CSI, Crossing Jordan or bones that have been representations of forensic chemistry. These chemicals not only microscopically examine and identify the blood or tissue matter, but also a variety of substances. For example, if researchers at the crime scene that someone has been drugged, forensic chemists can look through all materials removed from the crime scene trying to determine the presence of specific drugs. In fact, even in a drug bust, when a person is carrying a small or large quantity of a controlled substance, "drugs" taken as evidence must be verified by looking at their chemical compounds. Alternatively, a forensic chemist can evaluate various fiber samples, such as clothing or carpet, to try to identify the presence of someone at a crime scene.
Although many people in the work of forensic chemistry laboratories unique chemistry, some people work in the collection of field tests. The knowledge of physics could have a forensic chemist for the scene to observe the blood patterns to determine which accidental or intentional injury. Forensic chemists may work in scenes where explosions or fires have occurred, to try to determine what happened. As much as you can evaluate a scenario to decide whether the crime occurred, I might be able to rule out malicious intent by examining the patterns of fire, and looking for some chemicals associated with making bombs or fire.
Forensic chemists are trained in organic chemistry, so they can run blood tests and other DNA samples from the body to identify and implement toxicology tests. Therefore, looking at the matter from the chemical point of view to gather more information about a substance, person, or crime, for a variety of reasons.