HI -- Not really a trend going on this and there are many ways of treating the back wall of a bookcase . One thing that has come back and is very popular again is the use of mirror . Tinted or smoky or black mirror would look very nice .

go ahead and paint it a different color.
Doing so adds color and depth. it also keeps the bookcases from looking so monolithic.

Wrap a piece of card/foam board with fabric or adhere wallpaper for a non permanent change! You can also adhere fabric with starch.