وَقَعْتُ فِي ظُلْمَةٍ
I fell into darkness.
When the context concerns uncertainty, doubt, or confusion, physical darkness can represent intellectual disorientation or lack of clarity. It parallels the relation between light and knowledge.
فِي شُبْهَةٍ كَالظُّلْمَةِ
In a doubt resembling darkness.
The second phrasing openly states a simile; the first may use darkness metaphorically. The difference is not just the number of words, but whether the image is made directly present or explicitly compared.