How to mock my superclass in rspec

I have the following code:

class Blurk
  def initialize
    # horror
  end

  def perform
    # yet more horror
  end
end

class Grunf < Blurk
   def perform
     super
     # here some code to test
   end
end

I would like to test the code on Grunf#perform but I can't figure out how to mock Blurk.

In general you shouldn't do it. A better approach (IMHO) is to make some slight changes to your class definitions

class Blurk
  def initialize
    # horror
  end

  def perform
    # yet more horror
    exec_perform
  end
  
  protected
  
  def exec_perform
    raise "exec_perform must be overriden"
  end
end

class Grunf < Blurk
   def exec_perform
     # here some code to test
   end
end

and to test Blurk and Grunf separately (in Blurk tests you may create TestClass definition in order to confirm exec_perform is run as expected). In Grunf's test file you would just test the exec_perform method.