I think your black spot is most likely melasma
other probability is lentigines.
Whatever the diagnosis, you are on right way of treatment.
But you found all of them in effective for you..
On that situation you could try ...
1. Laser therapy
2. Intense Pulsed Light (IPL) or
3. Deep chemical peel
can select the perfect one for your face.
Dr. MM Mahmud (firstname.lastname@example.org)