It might just be Echoes of the Great Song by David Gemmell, but it would need a rather loose interpretation of worlds colliding, and also the date is slightly out as it was published in 2002. If your father would like to check there is a summary on the Fandom web site here.
The book starts on the world of the Avatars where crystals are used for power, but by the time the book starts these crystals are ceasing to work.
In the book the Crystal Queen teleports her continent from another world onto the world of the Avatars, which could I guess be worlds colliding. I can't remember if the Crystal Queen's world was another planet moving through space or some form of alternate dimension. Also I'm not sure if it's ever made clear exactly how this teleporting was done.
The people of the Crystal Queen, the Almecs (a thinly disguised version of the Olmecs), also use crystals for power but their crystals are charged by sacrificing humns and allowing their blood to soak into the crystals.
The book was published with lots of different covers. There is a list of all the covers here. One of the covers is mainly blue:
