Most places you'd normally enter a single block type in WorldEdit, you can instead enter one of these which will give you a random distribution of blocks instead of just one kind. You can specify which blocks and how often they should show up.
As an example, if you were to select a zone and enter the following command you'd end up with something like the following screenshot:
//replace grass,dirt 50%cloth:red,50%cloth:white
Note that you cannot specify a probability pattern for the blocks you are going to replace - only for what you will replace them with.
So to get dungeony walls...
Assuming you already have the stone brick walls laid out, you just want a brush that replaces some of the stone bricks with mossy or cracked stone bricks. The below commands will give you a brush with a radius of 5 that exclusively targets stone brick (all kinds, including mossy and cracked) and replaces it with 70% regular stone brick (
stonebrick:0), 10% mossy stone brick (
stonebrick:1) and 20% cracked stone brick (
//brush sphere 70%stonebrick:0,10%stonebrick:1,20%stonebrick:2 5
Just tweak the probabilities until you get an effect you like. You might want to make certain areas more mossy, or certain areas less mossy and more cracked, so play with it a bit.