Change the coolant out and regill with plain tap water...run until hot wth heater valve open....
Drain water when cool enough to work on
Refill with fresh plain tap water and run until warm...pressure test with heater valve open....
The balck discoloration is most likely too much radiator stop leak in the system...usually to stop heater core leaks prior to sale....
If heater core is leaking you can bypass it until repairs.
Prior to daily driving, drain water and refill with fresh clean coolant....
Dont let your pets get anywhere near the drain off from the original flush or the subsequent flushes...or the coolant for that matter...
