Betta Fish: Black “Stomach” on New Betta?

I got Cirrus yesterday from Petsmart. She is in my months old shrimp tank, loving the mosquito larvae outbreak. I noticed she has a dark “spot” behind her gills. It’s on both sides of her and seems to be more internal than a scale issue. Is this normal with white bettas? I’ve only ever had black bettas.

Tank Params:
Established: 6+ months ago, did a fish-less cycle for 2 months before that, shrimp for 6+ months, betta is new as of yesterday
Ammonia: 0ppm
Nitrite: 0ppm
Nitrate: 0-5ppm
Temp: 79F
Tank size: 5Gal (betta is 1” so it’s more space than it seems)
Meds: Kanaplex dosed yesterday for her fin rot

Just wanna make sure I’m doing her right and treating whatever she has.