Blood packs are simply packs of blood that were originally used for blood transfusions before the War. Now they have little use and can be found scattered about old medical facilities and in first aid kits.
Blood packs are large packs of dark red blood that can be found around the wasteland. They are found more frequently in Fallout 3 and in addition can be made more useful through the Hematophage perk. Otherwise, blood packs only heal 1 HP when consumed. In Fallout 4, a blood pack heals 50 HP when consumed.