It is a bug. For mine, I rcvd a severe weather alert text with high priority. If you rcvd any message like that, delete the text, then force stop your messaging or reboot your phone. Should go away.
Exactly what just happened to me. Severe weather warning text and then some contacts in my list turned bright blue. I deleted the warning text, rebooted, and contacts are back to normal color (white on black bacground). Thank you, raguna.