I guess he learned the hard way that even a man-of-God can't make a deal with the Devil without getting his fingers burned.
I can also only guess as to what he had in mind for Blade. Which was probably to make him thirsty enough, for blood, that he would be able to better empathize with what the ash-addicts were going through. And, thus, perhaps be more amenable to sharing his appetite-supressant with them. Sort of like methadone being used to combat the withdrawal pains of heroin and cocaine addicts.
But, evidently, Blade was thinking long-term. He knew that eventually the ash-dealers would hunt the source of their designer drug into extinction. Similar to what nearly happened to beavers and bison in the 1800's. Ergo, the ash-dealers would ultimately put themselves out of business. And, as a result, simplify the Rev's work.
Unfortunately, the Rev forgot that patience is a heavenly virtue. He wanted an immediate solution to the problem! So, he committed what he thought was the lesser of two evils: betraying Blade to his first victims. And, he paid the price.
Still, Blade might be gaining a potential new ally in the form of FBI Agent Collins. He has that German book recovered from ex-Detective Boone (which the latter, in turn, probably confiscated from Sands, the vampire cardsharp). That, in turn, might lead to a meeting with Blade, via Krista and the manhunt for Boone.
Only time, and further episodes, will tell.