i actually think that might happen regardless of wither it was intentional or not. like how pigs can only spawn on grass (in light), i think it'll be hell creatures will spawn from hell blocks, regardless to if they are in hell or not.
Well, spawning creatures near a somewhat common stone would kinda cause chaos and it would be somewhat difficult to program to make it work really well
I do like the bloody cobblestone though, I want that
maybe you could possibly harvest a mob spawner in hell and put it anywhere, that would be cool
Spawning creatures near a somewhat common stone is already in the game... That's what Mob Spawners are. A block that spawns monsters. The same concept could be applied when you take a block out of hell and into the real world. It just turns into a Mob Spawner with the textures of that particular hell block.
Also, I suggested in another thread (and someone else has also suggested something similar here) that hell blocks should kill off the vegetation surrounding it in some radius away. So grass would slowly die out to dirt and trees would so the same.
I like the idea of Hell Monsters slowly turning surrounding blocks into Hell versions. It just shouldn't be too fast though... don't want to drop off 1 hell block and have death be upon ye before the break o' dawn... as cool as that would be, it would ruin your save quickly should you not place a light or not think and place a torch (That would then go out in like 3 days or however long they last)...
-_-' there is only like, one mob spawner per dungeon, and there is way more mossy cobblestone than mob spawner, it's kinda like making the mossy cobblestone spawn monsters, that would be crazy, but I guess you could lower the amount of spawning, but I'd still prefer it to be just bloody cobblestone
Rather than having mobs spawn more hellstone, I'd prefer to see it capable of spreading like grass. Adjacent tiles have a small chance of becoming hellstone themselves. Dirt becomes helldirt, cobble becomes hellcobble, perhaps water even becomes lava... et cetera. The greater the ambient light the lower the chance of spreading (ie. the opposite of grass). The more hell materials you have in an area, the greater the chance that a mob spawned under dim light conditions will be a hell mob rather than a regular one. The hellstone doesn't act as a spawner itself, it just creates a chance that hell mobs will spawn in that chunk.