I’m trying to imagine in my head what touching a herpes lesion would feel like with my eyes closed. In the blister or ulcer stage, it would not feel rough or like pimples with a rough surface. With a scab, it might feel that way. However, what you describe sounds a bit more like it could be a genital wart.
But the blister does worry me slightly. Normally, a herpes blister would last only a few days in the labia, maybe a week, but three weeks? That’s a pretty long time for a herpes blister.
The fact that your test at 9.5 weeks is negative is reassuring indeed. A test at 16 weeks will be even more clear.
Have you considered seeing this person in daylight and looking at his penis or talking to him about your concerns?
And then finally, a think mutual masturbation is an extremely low risk experience. It sounds like his hands were mostly on you which is a good thing in terms of your concerns.
In summary, for many reasons, I don’t think this is herpes.