The branches were coated with a pitchy resin that was dark and sticky.

pitchy 是由名词 pitch（沥青、树脂）加上形容词后缀 -y 构成的形容词，表示"像沥青一样的"或"含有沥青的"。在这里形容树脂的颜色和质地像沥青一样又黑又黏。