High cirrus clouds depict in dark blue colours in the Night Microphysics RGB. The BTD of the MSG channels (IR12.0 - IR10.8) is used to detect cirrus clouds. The temperature range is chosen the way, that the red channel gives no contribution when cirrus clouds are present. The green colour beam gives no contribution either.
Though cirrus clouds are cold, background radiation from warm surfaces make the blue colour beam contribute a little bit. Therefore cirrus clouds over land or sea appear blueish. When cirrus clouds are located over high reaching opaque clouds or very cold land, they appear in black as the warm background radiation is missing (see image below: yellow arrows).