I also can see the merits of Firelance's proposal. I think it would not be unreasonable to use my effect for young black dragons and Firelance's for adult black dragons. This would actual result in a nicely organic effect where the black dragon is delevoping its Cloud of Darkness ability as it ages.
I think your version is actually a bit stronger than Firelance's, because you switched it to a minor. With your version, the dragon is attacking every turn, with CA on average half the time. With Firelance's, he's attacking every other turn (1 turn to activate the cloud and hide, the next turn to attack with CA).
Both ideas solve the problem nicely, but I prefer Firelance's version, for a couple reasons: first, your version is a bit of a unique mechanic to remember at the table; that's fine if it has unique results, but if you can accomplish the same goal with a more standard mechanic, it's preferable. Second, the hide-then-attack tempo with Firelance's version is closer to the original in feel -- people will still think of a black dragon as a turtler, without being annoyed by it like they were with the original.
I'm leveling up a young black to 7. I gave him 2d6 for CA (with melee only) -- I couldn't find any level 7 lurkers in either MM with a CA power, but I found a level 8 with 2d6. I also dropped stealth by 5, removed the sustain on the darkness, and bumped the size to 3 -- 6x6 seems like a big enough size to be effective without dominating the entire field.
Finally, following the MM2 pattern for solos, I reduced the HP to 6/lvl (the lurker standard) instead of the 8/lvl for all solos, dropped all defences by 2, and added an extra damage power that only works while bloodied: Acid Blood: immediate reaction (only when bloodied), any melee attacker that hits the dragon is splashed with blood for 1d6 acid damage.