The blue painters tape sucks. Its horrible. When the paint doesnt run under it, it builds a thick line between the two colors from the mask. Sometimes wont stick, or will stick to well thus ripping the paint. Also the tape is really "fibery", so it tends to have some color filter through.
Just grab a roll of the Tamiya Tape. I bought mine 6months ago and have half a roll left. It last for ever. Definately worth the what, five bucks I think. Great stuff.