Slate is also my weapon of choice for basing, check out my gallery, I reckon most of my stuff is slated up!
I would always ALWAYS reccomend painting anything natural that you use for basing, be it twigs/wood slate etc. The paint helps disguise its scale and also ties it in with the model better.
Demonheralds recipies work great, and I have also found that purple is also a great wash for slate too.