Cook the potatoes in a large pot of boiling salted water until they're tender, which will take about 15 minutes. Drain them and return them to the pot.
In a small saucepan, heat milk and garlic until hot (don't let it boil).
Add milk, blue cheese, and garlic to the potatoes and mash until potatoes are almost smooth. Let stand for 5 minutes to thicken.
Stir in rosemary and season with salt and pepper.