Blood packs are simply packs of blood used for blood transfusions and, initially, is next to useless as a healing item as it only restores +1 Hit Point per pack. With the Hematophage perk, blood packs will restore a total of +20 Hit Points instead of +1. Blood packs, unlike other basic medicines and chems, have a weight of 1 lb.
Blood packs are often found in first aid boxes, along with stimpaks, dirty water, purified water, and RadAway. First aid boxes 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.