Musta'ar lahu: the intended subject for which the borrowed word is used (the simile’s subject).
Musta'ar minhu: the source meaning from which the word is borrowed (the comparison model).
Musta'ar: the borrowed expression itself.
When “darkness” is applied to misguidance, misguidance is the intended referent, literal darkness is the source, and the word “darkness” is borrowed. Their connection is the difficulty of finding a way.
The borrowed word should not be confused with its intended referent.