The rival tried to blacken his reputation by spreading rumors.

blacken 是动词 black（使变黑）加上后缀 -en 构成的动词。black 本义是"黑色"，引申为"污点、负面"，加上 -en 表示"使...变得..."。blacken 字面意思是"使变黑"，引申为"玷污、败坏"。