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 ½ 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.