One of the newer features of LFD is that for the new instances, we have to discover them. Let's start with Blackrock Caverns, since that's the least-intuitive to get to. It's in Blackrock Mountain, so there are two ways to get there.
The first is a Mole Machine in Hyjal. If you're doing the quests there, you'll find a gnome (not a goblin: Shintar corrected my failing memory, it's a gnome), Finkle Einhorn, that says "This can only mean one thing..." and there will be a mole machine right next to him. I went back to take pictures, and he was phased out since I'd already done the quests. But he should be there (unless I'm clearly mistaken, which is possible), so if you could please pretend that the douchebag with the giant drake is actually just covering the NPCs, I'd appreciate it.
Mole Machine here:
When you zone in, you'll be on top of this platform here:
You'll want to go up and out, not down and out:
Keep going up:
And up the chain (you can mount up here):
Take a right after you've crossed the lava:
There will be a hallway to the right after this archway, next to the Summoning Stone.
At this pile of junk in the hallway, take another right:
The portal is in sight!
When you get close enough to the purple portal, you should discover BRC.
If you're just flying in from the Eastern Kingdoms from the regular Blackrock Mountain entrance, you'll want to follow the chain that's held by this dwarf (the one with the flailing hand).
Don't worry, this is by far the most confusing instance to get to. The others can reasonably be reached with a map location.