Your only restriction with naming cards is that you must name a card that actually exists in Magic. Blacker Lotus is certainly a card.
Why would you want to do this though?
actually, no, you're not allowed to name blacker lotus.
201.3. If an effect instructs a player to name a card, the player must choose the name of a card that exists in the Oracle card reference (see rule 108.1) and is legal in the format of the game the player is playing. (See rule 100.6.) If the player wants to name a split card, the player must name both halves of the split card. (See rule 708.) If the player wants to name a flip cards alternative name, the player may do so. (See rule 709.) If the player wants to name the back face of a double-faced card, the player may do so. (See rule 711.) A player may not choose the name of a token unless its also the name of a card.
any way to get Laboratory Maniac out on t1? t2 this. t3win?
The best part is when you name Dark Banishing in an actual legacy tournament, and forget that you sideboarded both copies out of the deck before the game started.
Sorry, I figured you were just playing 60-card-anything. Never figured you'd actually want to do this at a sanctioned event.
In fact, their copy of the spell will resolve before yours will, thus they won't even know what you are going to name until after they have named their card and revealed cards until they have found it.
From the Gatherer:
10/1/2008 - You don't name a card until Demonic Consultation resolves.
It's not part of the casting, it's part of the resolution. So your opponents will still be able to name any card they want.
if you have to CHOOSE, or NAME something, you do it as the spell resolves.
if you have to TARGET something, you have to name the targets as you cast the spell. this is always true when the word TARGET is used.