'Blood' is easily detectible and can be found in certain items long after it was deposited there. However, depending on the conditions of where the evidence has been deposited, it's value to forensic scientists (or the police), diminishes greatly, very quickly. So, the forensic scientist will be able to identify that a particular stain may be blood, or sweat, or saliva, or spermatoza; but whose it is and when it was deposited onto the suspect sample, may be impossible to tell after a relatively short time.
Locard's Principle is that 'every contact leaves a trace'. But blood can be cleaned up. It may be difficult depending on the surface it's come into contact with. Smooth, shiny, plastic surfaces will be easy to clean, whilst a fabric such as cotton will be very difficult to get rid of. But as stated, you may be able to determine what it is, but whose it is, is another problem.