Black Soul Gems are not called Black Soul Gems because they can only capture the soul of an evil creature.
They are called Black Soul Gems because they can capture the soul of a living humanoid.
And having your soul captured = eternal damnation, and so it's very, very, very evil, hence the evil name on the gem.
The Souls of humanoids, on the other hand, will always be of 'grand' status, even if you kill a weak, old grandma and trap her soul in your Black Soul Gem.
Black soul gems works like any other soul gem, except they can also capture the soul of a humanoid race, and those souls will always be Grand status.