Hey Sunmoon,
Well done on the info
I can honestly say that I can see only the white ones which normally disperse over time. My theory is (and this is probably NOT a new idea) that they spray overnight so by the time you're up and about to go to work/school/whatever, they're not as visible. I hear a few times during the week at night, some **** flying around, but I've never investigated this - too chicken to walk outside in the middle of the night!
I'll keep an eye out for those blue/black ones you've noticed on the weekends.