Wet Blue refers to moist chrome-tanned leather. In this phase, the leather is tanned, but neither dried, dyed nor finished. The bluish colouring is produced by the chrome tanning agent (Chromium (III) oxide), which is blue and is contained in the leather after tanning.
Nowadays leather is dyed through in multiple colours but the dyes have no opacity. In the case of bright leathers, the reverse of the finished leather retains the blueish colour of the chrome tanning agents. During retanning, the leather is bleached so that light colours can be achieved with better dying.