My nipple aches and feels like the milk is blocked. Is this a milk blister?

This looks like a nipple bleb. It must be hurting you tremendously. Here's what you can try: First, soak the nipple in warm water to soften the bleb. Then try rubbing it gently with a clean, damp washcloth until it opens or loosens. Next, feed the baby or express some milk to make sure any thickened milk is removed. You may see a small strand of hardened milk come out from the tiny duct opening.