Why does Messages require access to googleapis.com?

I recently set up my own DNS blacklist in an attempt to keep my local computers/mobile phone from accessing web content I don't consider trustworthy.

After I had blocked some Google services i noticed that Messages (on El Capitan as well on iOS 9.0.2) can't send images any longer. As I had also reset Llittle Snitch on my Mac I noticed that Messages contacts googleapis.com before sending an image and refuses to send if this service is not available.

Does anybody know what's going on? Does Apple use GCM services for implementing Messages? That would be outrageous!