Hot water boiler, indirect water heater, oxide, leaks...

A couple of weeks ago, my 5 year old boiler (hot water heat) started disgorging water through the relief valve (not to much, say about a quart per episode. So: called the plumber and they installed a new relief valve and pressure regulator valve, mucho dinero, and an expansion tank (though the old one was okay) which was too small and had to be replaced with one that is bigger than the one I had. At least they refunded about a third of the mucho dinero. But it was still mucho.
Anyhow, I guess the system was doing what it was supposed to do with the old equipment, because it is still doing it with the new equipment, except now there is black oxide in the discharged water.
The plumbers claim that I need to replace the 35-year old Amitrol - however for a test, I took the Amitrol out of the system and the boiler still sent out water through the new relief valve. There is an old leak in the pipe that goes to a bathroom, but it was never active until I had a guest who took a shower there - and afterward there was about a quart of water on the basement floor under the leak - full of black oxide.
I should mention that all the radiators in the house were converted from steam to hot water, at least 50 years ago, and tend to be cool on the bottom, maybe from sludge?
Is there any hope, or should I just start over with electric?
Thanks for reading!