Velgor already had assimilated a drop of the source blood of a Shadow Demon once, right in the beginning of his cultivation path and doing it again, would actually be detrimental to his health. More than a single drop of that blood, could cause many complications for a person, due to the differences in constitution between the beings. Actually, assimilating a second drop of source blood was so dangerous, that even at time like this, Velgor didn't think of doing it.
However, there was another way... Velgor didn't need to assimilate it in order to get more powerful, he just had to drink that blood. Drinking it, would make it possible for him to extract the power while not touching the source. The extracted power would add to his own for a brief moment and vanish after some time, increasing the intensity of his powers, but not raising the quality of it. All in all, it would act as a buff for the individual that drank it.