Blue platys, powder blue gouramis, turqoise rainbows, blue danios, neons. Define peaceful and size requirements. My JD is pretty blue and very peaceful for her type.
Blue diamond discus
Cobalt blue discus
Blue knight discus
Blue dolphin cichlids
male elongatus
jack dempsey
electric blue jack dempsey
demasoni
neon tetra
cardinal tetra
blue tetra
red fin blue tetra
turquoise rainbow
neon rainbow
L128
L239
frontosas
many more, but size, style and tank type will all be factors.