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Edit: There is a method for non-pet classes to solo the encounter. It takes longer, but generally has better success. (Thank you to Xertious from reddit!) See their video. http://www.youtube.com/watch?v=X8WKXuefoMs
For the most part, Blackwing Lair (BWL) is solo-able. The biggest hurdle is the first boss, Razorgore the Untamed. The mechanics of his encounter in nearly all cases requires a minimum of two characters. I've heard of hunters being able to solo Razorgore (probably because of their pets), so warlocks could probably solo it as well.
BWL does have some tricky mobs/bosses and an... interesting room. Some mobs are only vulnerable to physical attacks. A couple of bosses have annoying curses that last for 10 minutes - and one of which will blow up your character once its duration expires. That interesting room? Imagine the slow-motion epic walks in front of the camera that you sometimes see in movies. Now picture that in a raid. Across one huge room, up a flight of stairs, across another huge room. While hugging walls. On the plus side, a bit before or with Patch 5.1, the Suppression Devices were modified so that any class (not just Rogue) could deactivate them.
Despite these things, BWL is still popular for transmogrification runs and for the battle pets added in 5.1. Nefarian is also the best source for farming a number of collectible items and recipes.
First encounter can easily be soloed if you have long lasting HoTs. Throw as many on your self as possible and most of the mobs won't even touch Razorgore.
is this a raid or dungeon?
It is a raid. It required 40 players to complete waaaaaay back in the day.