You started to see the transition in the late 80's/early 90's with cats like Bobby Brown, Jodeci etc.
It's bigger than just perception tho. As hip-hop started to grow in the biz and mainstream it intertwined with R&B as far as "urban markets", hence all the mix of hip-hop and r&b especially in the 90's, to the point hip-hop outgrew r&b as far as mainstream dominence that r&b basically became hip-hop. But it started in the late 80's/early 90's, and fully blew in the 90's & then became what it became since..and i dont like it, honestly.
As far as now (Chris Brown, Trey Songz etc), they aint doing nothin new. But they basically have the rap personas but only sing - ie. Jodeci, esp since hip-hop grew. Even in marketing its the same.