I haven't lived in Seattle since 2011. I remember Belltown having one or two blocks that felt a little spotty or like you wanted your guard up, but the rest of it was thoroughly gentrified by the time I left, and it was a common choice to live for new college grads from the suburbs arriving to work in tech. I have to assume the process has continued.
Yep, it's even fancier now. The main spot where it seems sketchy is directly across the street from the Downtown Emergency Service Center.
The area around 3rd and Blanchard is sketchy, but in the rest of Belltown the biggest hazards are incompetent Uber/lyft/doordash/etc. drivers and drunk people on lyme scooters.