It looks like sunscald, is it in bright light

It looks like mechanical damage, a scrape perhaps?

When I noticed the spot I scratched a little with a swab to make sure that wasn't meal bugs but didnt change anything.

I place the plant in a place where is protected from directly sun, so I don't think that is sunburn either, also the leaf didnt turn brown or yellow.

I have checked on the Internet and books and I can't find out what could be :(

An abrasion to a leaf will cause that affected area to basically become dark, or bruised if not dead (that top layer). Cut a leaf or abrade it, and it becomes a dark scab.
It does look more like sunburn. It's not mealy bugs.
The effects of sunburn may not be readily apparent At any time, the plant could have been subjected to direct sun, and a scald.
If the area turns dark, an weepy (oozing dark fluid) then it's a bacterial infection...if so, then that part of the leaf should be excised to limit the spread of the disease, and spray affected area with Phyton or Physan to disinfect the wound. Ground cinnamon put on a wound will deter fungus and bacteria.