What can I do to keep paint from seeping under the 'blue' tape?

First, be sure that it is sealed against the wall really well. Are you painting stripes or lines or is this just for trim? For stripes, first paint a coat of the color of the other stripe - the existing base color. This ensures that any seepage blends in. If it's just trim, just get a small paint brush and do touch ups when you are finished. Good luck!

You can use a credit card or a similar type of card to smooth down the tape before you paint- this should stop seepage! Good luck! Keep us posted!

I read an article a long time ago that said to put clear nail polish on the edges of the tape. I guess creates a shield so the paint doesn't go under it. Good luck.

I really like the Frog Tape - best way to use it, is to apply the tape where needed and burnish the edge - then take a wet sponge and run it along the side that will be painted. Wetting the tape with water first activates the material in the Frog Tape and should prevent seepage. Best of luck!

Be sure the tape is pressed down well. Don't put a lot of paint on your brush when you're near the tape and brush lightly. If necessary, give the area a second coat after the first one is dry.