Hi and welcome to the site, I’m Louie. Sorry you had to wait awhile but I’ll do my best to help.
Initially, does the described "...black dot..." also show in any / all resolutions? ...any/all ISO/sensitivity range?
The described symptom then would be indicative of an internal hardware issue or more particularly, an artifact/pixerl fault with 16 Megapixel CCD sensor. The FH5 has a problem with its "eye".
Would the camera by chance still be under warranty?