Viable blood sample is a quest item in Fallout 4. Viable blood samples are used during the miscellaneous quest Blood Bank in which the Sole Survivor is tasked with bringing these samples back to Senior Scribe Neriah.
Viable blood samples serve no other purpose than to be sold for 50 caps to Senior Scribe Neriah. It appears to be a small test tube, filled with 'viable' blood and sealed by a bung at the top of the test tube. It is worth 10 caps if sold to a vendor, and worth 50 caps if sold to Scribe Neriah.
Viable blood samples can be found on the bodies of certain enemies. They seem to appear on any enemies that could be exposed to radiation, namely:
There is, however, no guarantee that a viable blood sample will appear on a corpse, and they do not appear in the game until the related quest is started.