Place the potatoes in a steamer basket fitted over a large pot of boiling water.
Cover and steam until the potatoes are knife-tender, 12 to 15 minutes.
Meanwhile, heat the milk and butter in a saucepan over low heat until hot and the butter is melted.
Drain the potato steaming water from the still hot pot and transfer the steamed potatoes into the pot.
Add the milk-butter mixture and mash with a masher to the desired consistency.
Stir in the blue cheese and season with salt and black pepper.