I was going through the abyss for some nat running, and I noticed a blood exit...can I actually craft bloods? I tried to go through it but it said a strange force blocked my exit. Can someone explain the blood alter exit?

It seems Jagex made exits for all of the runes in the abyss although not all of them can be used at the moment. They might be there for future use when blood crafting, soul crafting etc is brought out for players.

Yeah, ever since the release of the Abyssal Space, there were entrances for Death, Blood and Soul runes. Yet only Death Rune crafting has been released since then. So until we can actually craft those runes, the entrances are not 'working' :

Oh ok, thanks for clearing that up.