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.
Collecting and selling these to Paladin Tristan is probably the easiest way to earn money in the game as Super Mutant's are all over DC and can be collected even if you don't make the kill (your follower kills them for example) and as the blood samples are weightless you can collect many and sell them all together. If you haven't yet completed Reilly's Rangers quest by the time you get to collect these you can collect many samples during the quest.
