It's a very underused item (actually the least used, according to the statistics on the HoN website!) and is far too expensive for the benefits it affords. It used to be a solid pickup on heroes like Defiler and Soul Reaper but has since been pushed aside in favor of other options. Basically we're trying to "resurrect" the item so it's actually used again.
At the moment, the designers are still looking closely at Helm of the Black Legion. It recently received a major rework and thus they're keeping a close eye on it to see if it requires any more adjustments.