Playing Saturday morning coffee time catch up, as usual

My disclaimer is that I luuuvv BT in every shape, form and size

I actually looked for and bought a BT (possibly Tabac) City Bag based on the fact it was worn, scratched, looked like it had been stained , but short of coming apart at the seams or piping worn through. So there would be patina after rehab.
The darkening on your bag, as has been said, is most likely wear. What I thought was a stain over the flap was actually just from wear, as I have seen it in pics of countless City bags like mine. At first I couldn't figure out why they all had a “stain” over the flap like mine!
While it seems you were not looking for the patina I was. In real life, aka while in use, I find natural light to be quite forgiving. With use and periodic light conditioning it is possible the rest of the bag may darken , although I doubt will be completely uniform.
BT, and the neutrals can be tricky beasts, IMO. I have seen some rehab amazingly here.
I was fortunate enough to find an Oliver in BT about 2 1/2 years ago , online, insanely cheap, considering it was like new except for a scrape on the piping. I also found a 1997 BT Bancroft, also insanely cheap and in very very good condition 1 1/2 years ago. Both got a light surface surface cleaning and then conditioning with Lexol, which is very light. Because they both get used regularly, I noticed recently the Ollie has darkened a bit , the Banny not so much. Both do show a few tiny random spots/stains/discolorations, which I attribute as casualties of being working breeds I put to use. And also to oxidation. Cause I know prior to me buying them they were sitting in closets, not out and about like they are under my watch. They do not get babied, say, like my cornflower Ace, lol.
So I expect my BTs, and related neutrals to possibly darken, maybe some spontaneous spotting, with use and conditioning. YMMV.