Black Lotus is still very useful for making flasks even in Burning Crusade. It's still rare, and expensive.
I've learned a good farming system for aquiring the black lotus, and can collect usually abut 4 an hour this way.
On our server black lotus sells for 25-50 gold a piece.
One thing I love about farming black lotus is because there is some travel time between places it allows me to get up, do a little housework, grab coffee, sign homework etc, even watch a movie half heartedly.
Things to know about how BL spawns:
It spawns one time per hour if it is picked, in one of roughly 14 spawn points in 4 zones. What does this mean exactly? If you pick the one in Eastern Plaguelands there will not be another spawn in that zone for another hour, move on to the next zone. Make note of the time you pick the first one on your route, so that you know that you have waited an hour and don't waste your time checking empty spawn points.
How I do it:
I start in Eastern Plaguelands, so I go Shattrath to Isle of Que'l Danas and fly to EPL. From Light Hopes Chapel, I go toward Bowman hill and hit all the points between there and Strat. Run back down by Blackwind lake, and hit the spawn points by WPL and hit all the points between there and back to LHC. Once I find my Lotus, whichever spawn point it is at, I move on to Burning Steppes:
Once at Burning Steppes the route is a little more of a circle, thankfully. Starting from Morgan's Vigil I travel south and check Pillar of Ash spawn points, make my way up to the altar, and around Black Rock Mountain then east towards ogre mound and back to Morgan's. Once I find my Lotus, there are two options. I hearth to shat and either take the port to Darnassus, and go to Winterspring. Or you can hearth to Shat and Zephyr to Tanaris and fly to Silithus. Either way you have one long flight ahead of you between the next two zones. If you are a druid, you could zephyr to Tanaris then port to moonglade and save yourself a nasty flight. I personally like the long flights because I can actually do something else for a sec. So I go Winterspring next.
Once in Winterspring the circut gets a little fruity again. I start right by Everlook and hit all the nodes to the south first then work my way back up to the middle of the map, and hit west side and then lastly if I have still not found my lotus, I hit that one far up north. Most of the lotus I have found in this zone has been south of Everlook, but it is random which node it spawns at. Once I find the lotus in this zone I head to Silithus.
On the long flight here, takes several minutes this is when I hit the potty, refill my coffee, and run a load of dishes or something. Once here I hit the North West portion of the map first, work my way south, then run all the southern points and work my way north again along the eastern side of the map. Once I have my lotus for this zone, I either hearth back to Shat and start over or fly to Theramore and boat back to MH and start over that way, depends on if my hearth is up and how lazy I feel.
In closing I'm sure there is a more efficient path, but really efficiency isn't the key here, because the circut must take at least an hour or your first zone will not have spawned a new lotus. So the idea is to make sure dawdle a bit if you find a few quick lotus and are under your hour, pick a few herbs along your path for bonus gold.
I hope this is useful for someone.