Black Knights are only battleline for deathrattle allegiance, so you can't use them as battleline if your army includes a necromancer, or a mourngul, or tomb scorpions.
If you just want something fast that can run for objectives, and are unconcerned with fighting ability, you can take tomb kings skeleton horsemen instead. Or if you want something that's a bit more combat capable (not much, but a bit), then you can take skeleton chariots.
If facing something like this, I'd be tempted to try a cheesed out unit of necropolis knights. Maybe somehting like:
Settra
2x herald
2x10 zombies
6 necropolis knights
One of your heralds will eat 4 wounds, and Settra 1 to 2 more, off the thundertusk before you get to charge, but the necropolis knights will handily slaughter the thundertusk in a round. Counterattack from the frostlord and mournfangs should drop 3 to 4 necroknights, but unless they get a lucky double turn, you'll heal all or most of that up in your next hero phase between the heralds and the knights' own banner, and can start winning through attrition/recursion, as is the undead way, with necroknights whittling down the frostlord while settra charges in to cut down mournfangs.
Or, for a more offensive alternative, you could try:
Settra
Necromancer
2x10 zombies
corpse cart
6 necroknights.
You lose 2/3 of the necroknights recursion, but significantly increase their damage output. You could kill the frostlord in a single turn with this set up, but without the heralds it's even more essential to get to the thundertusk first to keep settra up, and you didn't need the help to down the tusk before, so I prefer the above, tankier approach.
The main downside is that the list is an obvious cheesefest, with a faction-leading special character taken in a small game, a legacy one from the old game who's long dead in current fluff at that. And it's an aesthetically displeasing list as well, mixing tomb kings and zombies, taking settra without chariots, relying on one of the silliest looking undead units GW ever put out, etc.
The ogres list, on the other hand, is thematically and aesthetically pleasing, all chosen from a single sublist. It's super powerful at this points value, but your opponent didn't have to be intentionally cheesing out to get there. Honestly, I have a hard time seeing how you'd end up with anything else for a 1000 point beastclaw list.