The blood puddle will still be there if they wish to examine it! Depending on how you want to time it, it may appear that someone else has been here and perhaps sampled a little of the blood already.

With some ingenuity, they may dig out a container from the lost and found and collect a sample of the blood. They will need to provide a distraction or do some careful sneaking to not get caught.

While the sample is currently contaminated, if they can find someone who would be able to possibly remove the contaminants and purify the blood, either through magical or scientific means (or a combination!), they may be able to provide an additional sample to the one that has been retrieved here.

As she is leaving, Ciri will feel a strange presence behind her. She can choose to turn around and look, or she may ignore it and continue on.