They were examining "flakes" of dry blood - not wet blood.
Contamination is being used as an excuse - so by doing that there is an implication that the labs were not as careful as they should have been.
And that does not even take into account the way the crime scene was handled.
Contamination of evidence with DNA by spreading minute amounts of the DNA around is different from contamination of a sizable amount of specific material. Minute amounts of DNA will not reveal what the tissue source of the DNA was so won't be able to establish that someone's blood, skin, saliva or other tissue was left. If you see a big ass stain and test it and fin the stain was blood based then you know you have a blood stain. Accidental contamination can't account for that blood. You can screw up determining whose blood it is if you are really sloppy and swap results from tests of two different blood sources but such is a different issue than contamination.
Contamination of blood occurs when wet blood is transferred somewhere by a means other than during an attack. It can occur before a crime or after. The transfer can be innocent and accidental or can be done intentionally. Intentional planting rarely works because so many things give it away including if you use a blood sample then it will have chemicals to preserve the sample and when such chemicals are detected it gives away that a stored sample was used.
Innocent transfers can happen from touching a victim to try to give aid or even walking through blood and thus leaving prints. This is why it is so important to be able to read blood evidence. Spatter doesn't happen from just coming upon a victim later you have to be present during an attack to get spatter on you so either will be a witness, fellow victim or the assailant.