Xr2I Heater Matrix / Coolant Probs!!

I have a 1991 XR2i. She has just had a rebuild (rings, bearings, gaskets, head was skimmed; all the usual things) I've had her back on the road for all of a day and already I have run into problems.

It started with my heater matrix showing the usual signs of it being on its way out (spitting water out of the vents and in the footwell). Today, after an 8 mile trip to town, she was spitting water out again. When I stopped I checked the coolant level and it was practically empty. I filled her back up with water (only water, I didn't add any bottled coolant) and took her home.

When I got home I immediately checked the coolant level and, although it hadn't dropped, it had changed colour - black!! (It was all black, not just a black oil film floating on the top). And there was water being pumped into the bottle from the pipe at the top of the bottle (I'm presuming that this is the pipe that sends the water round the heater matrix).

So I left her for approx an hour to cool off and checked her again. The coolant level hadn't dropped but had changed back to "normal" coolant colour, blue.

Can the coolant turning black be a contamination from the faulty heater matrix?? I have considered the possibility that the head gasket is faulty, however, theres no milky residue in my oil and she is showing no signs what so ever of a head gasket problem. In fact she is running as sweet as!

She was running a little on the hot side on the way home, but she didn't get into the red.

Oil levels are fine and havent moved.

Any suggestions??