Lack of sensation in right forehead, temple, upper cheek for 5 months? Brain ct normal and cervical + lumbar mri normal, blood test normal. Cause?

Trigeminal neuralgia. You may have trigeminal neuralgia. The Cat scan should have a clear view of the meckel's cave area in the brain. This is the area where the nerves that innervate that area run through. You should see an ent doctor to follow this. Good luck and feel better.
Sounds like. a NEURITIS of the TRIGEMINAL Nerve...

This will not show up in a brain MRI

Mention to the physician who ordered the testing you have described!
Hope this is helpful
Dr Z.