...Bocch does use Kamui though, not to enter the situation outside but to enter a dusty room deep in the hideout. -
(5d6h) <Kusa > Academy Student Bocchi - The room is pitch black but Bocch snaps his fingers and several sconces of flames ignite in the four corners of the room and around an alabaster platform with the symbol of Jashin carved into its face. Bocch would walk forward and ascend the -
(5d6h) <Kusa > Academy Student Bocchi - several steps that lead onto the platform. He would walk across its surface until he stood in the middle of his God's symbol...
...The Jashinist opens his mouth and catches the drop on his tongue, swallowing it, and when he does his skin immediately begins to change. His -
(1d15h) <暁> 真 Bocchiere - entire body turns pitch black and white markings symbolizing a skeleton would appear over the blackness. The dais where he stands is actually where Kamui once died, and the marking of Jashin on it is stained with the Otokage's, among others, -
(1d15h) <暁> 真 Bocchiere - blood, thus the ritual is complete...
...In some instances, small, self-inflicted harms will produce blood so that the summoned can use summoning techniques,[62] for an example, although greater harms won't make the summoner bleed...
Unless I missed something, if Bocc already cursed Kamui before, then it was entirely unecessary for him to acquire more of Kamui's blood, unless becoming an Edo Tensei zombie nullified that, which is unlikely since Edos can still have blood drawn from them, though only if the injury is self inflicted apparently.
Of course, that's not the main issue being debated here. What is the main issue is whether the mark would still be viable after all of this time. The room is not completely free of the elements, otherwise, it would not be dusty. So, presuming that it has been months IC since the marking of the seal, it is not far-fetched to say that the blood may have degenerated.
However, the 1st and 2nd Hokage were brought back via Edo Tensei; I presume that some of the bloodstains in their armor were used, if i recall correctly. Those two farts died decades before Orochimaru brought them back.
Of course, it can be argued that it was merely dead cells that were used, not blood per say, in which case, I will have to keep looking for the chapter/episode that went into details about how Oro brought those two back.
It is surprisingly difficult believe it or not, since it isn't an episode highlight, nor does the wiki explanation of the situation provide any other citation other than Kabuto's. I watched that episode and, frankly, all it did was tell me that a fixed portion of flesh containing DNA works, which does not necessarily mean blood.
Long story short, the blood may have degenerated after all of this time, simply because the room is dusty. Dust can be a rather destructive thing over a long period of time, so regardless of how long the hideout has been in use by Bocc, dust accumulation might would damage the validity of the marking. Of course, it would not be much of a chore to restore it with some fresh, undusty blood.
That's my two cents on it.