IIRC, "Prestidigitation" has a duration (1 hour?), which probably means that anything you did would only last for that duration.
Personally, i don't think a drop of (tainted) blood from the scene is enough for a successful scry. If it's ruled as enough, there are a number of ways to prevent the mage from finding your guy (other than the ones you mentioned):
- place him in an extradimensional space, such as the Magnificent Mansion, during the time of the scrying.
- Mage's Private Sanctum
Or, for some more "down to earth" solutions:
- sabotage the crime scene to prevent collection of evidence (ex.: shoot a fireball into it)
- destroy/replace the evidence and/or blood samples before the scrying (it's amazing what a few well placed "Dimension Door" and "Invisibility" spells can accomplish
)
EDIT: Just remembered reading this advice somewhere, so i can't take credit...
- Put the guy in a room lined with lead.