[Haskell-cafe] Flexible instances

I'm a little confused. Why is this allowed:

data Blah = Blah

instance Eq Blah where x == y = True

But not this:

class Stringable a where toString :: a -> String

instance Stringable [Char] where toString = id

(Resulting in:)

Illegal instance declaration for `Stringable [Char]' (All instance types must be of the form (T a1 ... an) where a1 ... an are distinct type *variables* Use -XFlexibleInstances if you want to disable this.) In the instance declaration for `Stringable [Char]'

'Blah' isn't a type variable, is it? Is my brain just not working right today?