SMP looks blue in photos, but it looks bluish black in mirror. Why?

All tattoos or SMP can potentially look green or blue under a bright light.  The degree of how blue /green it may look depends on the quality of the pigment.  Even the best pigment will look slightly blue/ green.The scientific reason for this is due to your skin's increased absorption of red light.  If you take away red from black, then it will looks slightly green.  (Your veins are not really green.)

The darker the pigments that are used, the greater the tendency to turn blueish. We use pigments that generally don't turn blue unless we use straight black which we don't do.