Interesting point.
Even if they are soap residue, they may be splatter from hand-washing and have trapped some epithelial cells.
Certainly along with other possibilities may be Knox had a bit of gingivitis a common cause of bleeding from the mouth especially when cleaning the teeth. My non expert opinion is that there also appears to have been water drops falling on the blood drops subsequently suggesting the blood was not recent and the basin had been used since the blood was deposited.
The tap is obviously dirty and had not benn cleaned by anyone (meredith or Amanda) for many days for all the talk of comparative cleanliness.