blood can freeze, right? I think I remember having blood (or was it milk) freeze into a glob and being able to cook it?
either way if it can't be cooked, liquids do indeed freeze if you leave the barrel say outside in winter, if blood does freeze, you could try:
1) if you can cook the frozen blood globs
2) dump the frozen blood globs into a pit, to free the barrels and/or make a blood contaminated water pool, you can drop the liquid but requires forbbiding the liquid and moving the barrel to a trade Depot when there's a caravan in town.
3) "recycle" it to sell back to caravans...
... let me explain: items that haven't been produced in your fort are marked in white and between parenthesis "(blood)", items produced in your fort, doesn't have the parenthesis and are coloured brown, a usual industry is to buy out the caravans and/or loot invaders and turn the crap they bring into brown "fortress-produced" items, this can be accomplished by adding value to them or transforming them in some way, usually decorating, sewing cloth/leather images on clothing/armour/goods, encrusting with cheap gems/glass to train up your jewellers, studding with metal, melting items for the metal, etc...
so back to the blood barrels, you can encrust them or decorate with bones, and the blood itself can be turned into brow "fortress-produced" if you let it freeze and thaw, freezing counts as a type of transformation on the item and it becomes brown... not that it matters as for value or anything, but if the blood barrels were seized from a caravan they'll be marked in red and you won't be able to resell them, unless you "brown-wash" them as explained...