Black flow pipes in underfloor heating?

Good morning, we took over our house from the GU in November 2021. Now all the flow tubes of the underfloor heating in the house are black.

Is this normal?
Do we need to possibly make a defect report to our GU / have it repaired by the sanitary installer?

Thank you very much for your help.

Yes, do that. They will say it doesn't matter, but maybe you will be lucky.

A sludge separator is needed, which is also used. The system would have to be flushed and filled with demineralized water (not with tap water).

Thank you, is there any information anywhere that it does matter? The complaint has been submitted. Let's see what comes.