So I had a chance to give some serious attempts on Heroic Blackhand tonight with the group finder for a guild run missing a tank and all I have to say is wow. The fight is split into 3 phases and the 1st phase isn't very hard but the 2nd feels like if you had to do Phase 2 of Heroic Garrosh and a super amped up version of the adds in Kargath at the same time. Lots of adds and the damage the raid takes ramps up awful quick, requiring a very high DPS output to avoid being overwhelmed.
Phase 3 itself isn't overly complex but requires a lot of DPS to get through before running out of space on the platform, which means you have to get so good at phase 2 that you start phase 3 with everyone up with cool downs ready. The damage output is even more intense and means healers have got to still have their cool downs as well & likely have to plan them out so one is always up for the high damage points.
At best, we got to phase 3 a couple times with just a few people dead but ran out of room before downing the boss.
Overall, this is a very interesting fight. It's much shorter than the Imperator fight but also a lot more intense. Like the Garrosh fight, Blackhand boils down to one colossal DPS check; can you get through Phase 2 quick enough to avoid the adds building up, can you get through phase 3 fast enough to not run out of room, and can you do it all without any mistakes. I can see why the elitists over at the WoW Dungeon and Raid forum who normally don't even consider early Mythic fights "hard" consider this to be a tough encounter.