How to relate to type from outer context

This is possible with the ScopedTypeVariables extension. You need to use explicit forall’s to bring the type variables into scope.

blah :: forall a b. a -> b -> a
blah x y = ble x where
    ble :: b -> b
    ble x = x

Trying to load this definition with ScopedTypeVariables enabled gives:

foo.hs:2:16:
    Couldn't match type `a' with `b'
      `a' is a rigid type variable bound by
          the type signature for blah :: a -> b -> a at foo.hs:2:1
      `b' is a rigid type variable bound by
          the type signature for blah :: a -> b -> a at foo.hs:2:1
    In the first argument of `ble', namely `x'
    In the expression: ble x
    In an equation for `blah':
        blah x y
          = ble x
          where
              ble :: b -> b
              ble x = x

You can tell that GHC interprets the two bs as the same type because the error says that a and b are bound on the same line.