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.