Loss of heater voltage for blue gun of CRT, failed heater filaments in CRT for blue gun, loss of blue drive caused by failed transistor or other similar fault.
If it's a bad CRT, it's not worth fixing, if it's a simple small failure, a shop could fix it pretty easily. Be sure to reset all the color controls first.
Sometimes a gentle whack will fix a problem like this if it's a poor internal connection.
http://www.repairfaq.org/samnew/tvfaq/tvintmisc.htm
Philco, February 2008
