For me, you really need to do something to bring up the background exposure to make these "blue hour." Right now they look a lot like snapshots taken at night because the flash is direct and on camera and the background is so dark.
Try bringing your ISO up, even to 800-1600 if needed. This will bring up the backgrpund exposure and convey the mood I feel you may have been going for. It will also allow you to work with less flash power.
You also *REALLY* need to get a cheap off camera flash if you wanna learn portraiture, especially portraiture in lighting like this. You can get a speedlight and wireless trigger combo for well under $100 these days. You clearly understand the need for this, but I couldn't go without saying it. It makes such a big difference.
We were partly under a bridge, it really was that dark there by that time. He wasn't willing to move much. I did take a few at other ISOs.. but there was so much noise it wasn't pretty and my subject was washed out. It might have worked with me as the subject, but I have a very different skin tone. How would you counter noise and washed out darker skin? And thanks!
I so rarely get to shoot people it isn't funny. In this case, this was very much unplanned. Even if I owned a flash, I wouldn't have had it on me. I do see a lot of wedding/prom type photographers with bigger flashes on a hot shoe in this location. Would one of those work? They look small enough to tuck in my bag and forget about..
On camera flash can work in a punch for this stuff. But if you can fit the flash in your bag, you can also tuck a wireless trigger in there is no problem. They're tiny.
The key to interesting and pleasing light for portraiture is getting the light at a different angle and height than the camera.
What kind of camera are you using?
Noise from high ISO shouldn't detract too much from an image like this, especially because noise primarily only shows up in shadows. Your subject should be well exposed with flash and therefore (using any DSLR made in the last 10 years) noise on the subject at ISO 800-1600 shouldn't really be a problem.
You mention the subject being washed out at higher ISO... the response to that here should be to lower the flash power to compensate. You'd do this because flash power is the only setting you can use to darken the subject without affecting the ambient exposure.