Blood pack

A blood pack is simply a pack of blood used for blood transfusions and, initially, is next to useless as a healing item as it only restores 1 HP per pack. Blood packs, unlike other basic medicines and chems, have a weight of 1 unit each.

Locations
It can often be found in searchable First Aid Boxes, along with stimpaks, dirty water, purified water, and Rad Away. First aid kits are commonly found in bathrooms on the interior wall next to the door. Blood packs can also be found in searchable IV stands in hospitals and clinics.

Hematophage
After the player acquires the Hematophage perk, blood packs become much more useful, with one pack restoring 20 HP with no radiation damage, making it just as useful as purified water, which is harder to find. You can earn this perk by talking to Vance after completion of the quest Blood Ties.

Unmarked quest: I Want to Drink Your Blood
They can also be traded to Vance and the Family in exchange for 15 caps if he made a deal between the Family and the people of Arefu.