Painting - paint cutoff on rounded wal corners question

Any suggestions on how to best do a straight cutoff along the protruded corner/edge of a wall that's rounded and textured? I have a small hallway leading into the bathroom that has no door, and I want to transition from a light color and white ceiling into a light blue for the bathroom. however, I'm having a problem getting the transition straight. This part is notoriously hard to paint straight with a brush, and tape won't seal against the rough surface.

Is there an alternative? or am I stuck with doing it by hand?...

Rounded corners are always a challenge when changing colors.
Marking a straight line works best, sometimes it helps to lightly score the line, barely cutting into the texture and the brush the paint to the line.

This is one of the few times, I, a typical DIY, would use tape. Maybe some of that "Frog" tape would work here if the existing surface was smooth.

However, I would NEVER place that (or any tape) down on any painted surface less than a month old.

SirWired

I always run into trouble when I use tape for this. Although, I have used Frog Tape and it is awesome tape. Another option is to use a BlendMate (http://www.blendmate.com/video.html) -- I'll never paint a color change the old way again. This tool actually blends the two paints together so there is no distinct line. It's easier than fussing with tape, too.

Wow what a novel idea. Looks like a product developed by an interior decorator.

My last project, after painstakingly trying to determine where to put the paint line on the bullnose corner with the wife, there was still more tedious work ahead. Because my walls are heavily textured, I've always had to score down the tape really well, paint, and remove tape immediately. After that has dried I followed up retouching the line with a sponge brush. I am definitely going to have to try this product out.