For the Blood Magic and Thaumcraft mods in Minecraft, how do I get a Transcendent Blood Orb?

Is that the orb that holds 70 million lifepoints? I'm not entirely sure on this. But try using a gluttony shard on th altar.

First, important note, the two mods alone do not allow the Transcendent Blood Orb (which I will refer to TBO). Instead the mod Forbidden Magic, which requires Thaumcraft at the least but also adds cross-over support for several other magic mods, Blood Magic included.

It is an infusion recipe, so first you will have to research the TBO in the Thaumcarft research book. I'm not aware if there's a research prerequisite, but it might involve scanning an Blood Magic orb.

From there you can mostly use the research book to tell you about everything you'll need. This is a pretty end-game research however, because I know several of the things included in ingredients are mutli-stage researches themselves. For instance you have to have a corrupted pick to get the Gluttony Shard.