I'm gonna throw one out there and say the temp gauge gets it's reading from the temp sensor.

not the temp switch (that operates the fan and the idiot light)
blk /blk ? humm. that just sounds like a ground.. doupt that is your problem... but grounding it off to a good source is a safe thing to do.
Your problem is the temp sensor on the side of the cyl head near the coolant fill spount. contacts are either corroded or the temp sensor is shot. The engine also uses this reading for fuel mixture.. you must get a check engine light? or poor fuel millage?
Good luck!
Eric