blue screen

I've noticed that there's a blank blue screen appearing on MBP. It shows up before you get right into the password security on startup and after the shutdown order. It only lasts for 2 seconds but Im still bit worried coz I don't remembering getting blue screen before. I tried using safe boot coz I've read that it may remove cache that may cause appearance of blue screen on startup but Im still seeing the blue screen. Anyone experiencing this? Does anyone know how to fix this or is this totally normal? TY