I'm having the exact same problem. Blue channel doesn't go through the dock. In this case the monitor is an LG E2241 and the Thunderbolt connection is going through an HDMI adapter. The monitor works fine plugged directly into my MBP 13".
I stopped buying Belkin products a few years back after suffering through janky router after janky router. Finally decided to give them another chance, and they did not disappoint. This dock is going back to the Apple Store this week.
EDIT: Tried the same (HDMI --> Mini DisplayPort --> dock) chain on an ASUS monitor. Same problem.
I've been having issues with my dock and the ethernet port, and have had little luck with electronic communications and their support. Have plans this week to try to phone them to see if that is more effective.