I actually agree that it would be unwise for a member of the Inquisition to be a KNOWN bloodmage.
While it is true that it is not inherently evil, it has the greatest potential for corruption - after all there is
a reason all powerful and evil (or at least completely ruthless) mages use it if they can.
And even if you just compare it to a gun, remember that in most countrys weapon access is heavily restricted
-for good reasons, imho.
Another, even more important reason would be PR. It doesn't matter if blood magic is evil, if most of
the people deem it such. The chantry forbids it (including Tevinter mind you), the dalish and many within the circle
frown upon it. And the vast majority of "normal" people are afraid of it.
Remember the keep dlc, when Levi Dryden realises that his ancestor told Avernus to use bloodmagic.
He was saddened that she did it, because it is evil.
The Grey Wardens? They don't care. They allow everything as long at it works against the Darkspawn.
They take murderers and traitors in their ranks if they are deemed competent. The Grey Wardens are, to
say it in ME words" completely Renegade.