I've got nothing to help, but my wife had the same issue a couple of days ago. My son sent her several photos in various formats via SMS in Hangouts and they were all black. He sent her the same pictures via email and they were fine. So, not picture problem. We haven't seen the problem since. But, then, we haven't gotten any pictures since then, either.
EDIT: also, we're all on Nexus 5s. So source and destination for the photos was a Nexus 5. My son is on T-Mobile and we're on AT&T.
EDIT 2: Let me modify that. Our son isn't on Google Plus, so our messages through him go SMS or MMS (not a pure Hangouts message). Normally, we use SMS to text him. But, now that I think about it, I don't think a picture can go via SMS. I'll double-check when I can look at the wife's phone. But, I guess it was an MMS message and not SMS. I also just took a picture with the phone and sent it through MMS to the wife, son, and daughter-in-law (all Nexus 5s, the wife and I are on AT&T, the son and daughter-in-law are on T-Mobile). The boy has not yet responded, but everyone else sees the picture fine. I'll have to have him send a picture to us and see if that works now.
EDIT 3: Except for our son's Nexus 5, we can all send and receive photos fine via Hangouts. My son's pictures come across to my wife and I (on AT&T) as black. His wife see's the same photo fine (both on T-Mobile). So, I thought it was a cross-carrier thing. But, the daughter-in-law's photos come across to us fine. I don't know if it's related, but he's the only one of us not on Google Plus.