I've read Blackmark before when it was in Savage Sword of Conan and in Marvel Preview but now I've finally read it as it was intended in the Fantgraphics edition that I bought a year or so ago.
To be blunt, it's not very good. It is full of terribly flowery prose and worse, the art isn't very good either. Gil Kane's art actually declines in quality as it goes on - I am guessing this is due to it being reproduced from copies of copies - but still not his best work - he certainly wasn't a great inker. An interesting historical item but that's all.