The super mutant blood sample is an item added to Fallout 3 in the Broken Steel add-on.
Not surprisingly, it is a sample of super mutant blood used by the Brotherhood of Steel to research a super mutant megaweapon.
When dropped, it has the same world model as a blood pack, but is weightless.
Unmarked quest: Blood Brotherhood
After beginning the Blood Brotherhood quest, super mutant blood samples can be found on the bodies of dead super mutants and sold to Paladin Tristan for 50 caps and 10xp each.