A very common issue that we encounter is rust in heating systems. Generally, when a sample of heating water is inspected, and it appears black, that is iron ferrite. This occurs over time and provided it has not been left too long to the point which it becomes sludgy then it is not detrimental to the system.
If a heating system is performing badly then it is likely that it is also running uneconomically. Often a small amount of maintenance is all that is required to restore a plumbing or heating system to its full capacity. The condition of many systems deteriorates over time especially if they have had an extended period without maintenance.