Today during the rinsing process of descaling a Rocket Giotto I opened the steam wand and this really dark black water came out. I would say about 3oz. It got clear eventually, but I've never seen that before. I have descaled this machine several times and I have never seen that. It only came out like that from the steam wand. The hot water wand was clear. Any ideas what that is?

I had a similar thing some days ago; see Particulates from boiler after descale problematic?

Apparently in my case it was probably scale coming loose as a result of the descaling. In my case (HX machine) the water ran clear from steam wand and water spout after some rinsing, but if I took water from the water spout directly after re-heating the machine I'd get more particulates again. It seems to be that a load of particulates is now lying at the bottom of the boiler, and every time I heat it up those particles get agitated and some may come out through the spout.

What I don't understand about your case is why the steam wand would run black water, but the hot water spout wouldn't.. I would expect it from both. Maybe it was already diluted enough, if you flushed them in that order? What does the water look like if you let the machine get cold, then heat up to operating pressure, then directly take water from the spout?

I did a lot of searching around for this and most recommendations eventually came down to opening up the boiler to verify what's going on and clean it out properly by hand..

Basically. I did a full descale on a 2 group expobar, which lead to the same problem you are encountering (from hot water/steam wands) and dreadfully black water that was impossible to completely flush. At some point in this process the autofill malfunctioned, running the boiler dry and destroying the 3700w 240 element. At that point we stripped the machine down completely, and soaked the boiler and all the copper in a descale/acid bath. For a comparatively clean machine, there was some areas of nearly clogged pipes that I might not have checked (the U return to one of the flowmeters was pretty close to blocked) and it was very nice to have the boiler completely removed for cleaning. Expobar works great now (although I need to replace the second solenoid/valve...) and thanks to Dale and Sebastian at Great Infusions for parts and advice. (I've been meaning to post up a thread at some point - but the black particulate problem rang a bell)

Good luck.