Insanguination

Let's have a look at Insanguination - or is that Exsanguination?

The 2nd Ed. Codex: Angels of Death (Rick Priestly & Jervis Johnson) gives the first look at this:

All Space Marine Chapters use gene-seed to trigger and control the processes that transform an ordinary mortal into a Space Marine. The gene-seed is encoded with all the information needed to reshape ordinary cell clusters into the special organs Space Marines possess, and it contains viral machines which rebuild the body to the template contained within the gene-seed. However, from the beginning there was no set way to activate the gene-seed.

Indeed, at the time when the Chapters of the First Founding were created, the process was still highly experimental and many different ways of controlling and managing the change were tried. This led to the Space Wolves using the ritual known as Blooding, the Imperial Fists using the process known as the Hand of Faith and the Blood Angels using Insanguination.

The Process was originally triggered by injecting the Aspirants with tiny samples of their Primarch's own blood. This practice of course ended with the death of Sanguinius. However, some of his blood was kept and preserved in the Red Grail. The living blood could not be kept this way for long and it was injected into the veins of the Sanguinary Priests. In this way they became living hosts to the power of Sanguinius. To this day, drinking the collected blood of the assembled Sanguinary Priests fromt the Red Grail is part of the ritual of creation of all Blood Angels Priests.

Looking at Adeptus Astartes for the Blood Angels by Phil Kelly, he adds numerous details to the history of the Blood Angels:

At the time when the First Founding Chapters were created, the Emperor himself oversaw the process of transferring gene-seed from Primarch to Space Marine. However, since the Emperor's interment in the Golden Throne, each Chapter has had a different method of controlling and managing the change. The Blood Angels originally practised Exsanguination, a process initially triggered by injecting aspirants with tiny samples of the Primarch's blood. Alas, this process ground to a halt after Sanguinius's death, but fortuitously some of his blood was kept in the relic known as the Red Grail. This living blood, even possessed of such incredible power, could not last for long in an unprotected state. Thus it was that the vitae of their dead Primarch was injected into the veins of the Sanguinary Priests. They became living hosts to the power of their Primarch. Even today, drinking the blood of the assembled Sanguinary Priests from the Red Grail is part of the ritual used in inducting new Blood Angels Priests. In turn, it is from these custodians of the pure lineage that the blood given to aspirants is taken.

The Phil Kelly fluff doesn't actually use the word Insanguination. It describes what happens to new Blood Angels recruits but doesn't use the word. The Priestly and Johnson fluff use Insanguination to denote all ways of the BA gene-seed being activated, while the Kelly fluff only uses Exsanguination to denote the activation of BA gene-seed taken directly from Sanguinius.