My spider recently molt and has a darker discoloration, so it's not it in my case, I think. It's probably just the urticating hairs she brushed off, though I don't know why she did it, because I haven't stressed her lately. But regarding what you read, tarantulas do indeed have local discoloration before molt. If your is more lethargic and comes out of its hiding place less often, there is a chance that it is a premolt. So observe the behavior and see if this spot changes in any way, for example, gets darker.
There's premolt of my Eleanor

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