We've had Evil Underdark Elves and Evil Underdark Dwarves, here's the obligatory Evil Underdark Humans and Halflings!
All dark ones are humanoids. They share Light Sensitivity, Blindsight, a racial Hide bonus, and some amount of sneak attack. In addition, they have the poorly-written Shadow Cloak ability, which gives them some concealment in light, and even more in shadows or darkness. I'll be assuming this ability functions thrice per day for a single round, but it's not too clear.
The 'lesser' variant, with 1 humanoid HD, small size, and an impressive +2/+6/+2/+0/+4/+0 stat spread. Dark creepers also have +2d6 sneak attack, a point of natural armor, and Evasion.
The obvious point of comparison is a rogue, and the dark creeper compares very favorably. High stats, Shadow Cloak, Blindsight, multiple free dice of sneak attack and Evasion are essentially all the good parts of a low-level rogue and then some.
I think a LA of +2 is completely fine here: a dark creeper rogue 1 will be behind on a rogue 3 in terms of HP, feats, and skills, but it'll have better damage and stats, as well as Shadow Cloak. That's a reasonable trade-off that shouldn't invalidate either option.
3 HD, and even more rogue traits. 3d6 sneak attack is still more than an at-level rogue would have, and it's augmented by Shadow Cloak, 3/day Fog Cloud as a SLA, Poison Use, and even better stats than the dark creeper had (total stat boost: +24).
Humanoid hit dice suck, but this should still get +0 at the very least. I'll even be safe and go for +1 for now. Do discuss!