As he fell, the scrapes on his hands were slowly bloodying his white shirt.

bloodying 是动词 bloody 的现在分词形式。bloody 由名词 blood（血）加上后缀 -y 构成，意思是"使沾上血"或"染上血迹"。在句中表示手擦伤处的血正在弄脏白衬衫的持续过程。