Reddit mentions of Hear It and Sing It!: Exploring Modal Jazz

Changing genres is a long process. It starts exactly where you're starting - imitating singers and sounding bad. That's ok!

Instead of whole-cloth imitating them, why not analyze what each singer does? Like this video: https://www.youtube.com/watch?v=dAhs35-xDOM&t=825s. Then try to mimic just one or two of those stylistic choices like a fast or slow vibrato, a short run or turn, sliding between notes. Just pick one thing of your favorite singer and perfect it. The things you like and make your voice sound good will be your style.

I would avoid modern jazz singers for now...go back to the 30s, 40s and 50s. There are a lot of modern "jazz" singers who sound very hokey and unnatural (what you want to avoid!) so don't get into the habit of singing like them!

To sing jazz specifically, you're going to have become a theory nerd. Good jazz singers manipulate rhythms, pitch, vibrato, scales, etc. You can start here: Judy Niemack: Exploring Modal Jazz. This will get you used to singing the modal scales and experimenting with improvising. Not all jazz singers improvise or scat, but they do understand the harmonic and melodic structure of the songs they sing.

For R&B, again, pick an artist you love. But also practice runs and turns, back and forward phrasing, and try a little mixed-voice "belting."

For blues, everything above plus learn the blues scale.

Good luck!

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