Blotchy means covered in uneven patches of colour. A face gone red in irregular blooms after crying, or after cold.
Skin is usually what is blotchy: blotchy cheeks, a blotchy rash, blotchy legs after a hot bath. It stretches to anything patchy — blotchy paintwork, a blotchy print where the ink went on unevenly. Behind it sits the noun blotch, a single irregular patch, a close relative of blot. Blotchy is never a compliment: the neutral word for the same thing is patchy, and the flattering one is dappled.
From blotch + -y.
Etymology from Wiktionary, CC BY-SA