There has already been a posting about black water but my situation seems to be different.
The black water I get is not predictable at all. It just will have a wierd black sediment come through the taps (usually in the bathtub) in a cloud that eventually clears up. Once it does clear up then there is no more hot water instantly! If you wait for about half an hour then the water warms up again and it's fine. This happens very infrequently. I have not been able to find anything that preceeds this happening. Sometimes morning, sometimes night. It can be winter, summer or whatever. Sometimes there'll be months between clouds but other times it will happen two or three times a week. It's all very confusing. This is an older four-plex that is a rental so I have no idea how long the hot water tank has been there. Landlord doesn't seem to care...

When it happens in the bath tub is there hot water at any other tap?

No there is no hot water anywhere in the house. The hot water tank is beside the bathroom so I figured I see it more in the bathtub since it's closest. The kitchen is on the floor above, so it'd be harder to get sediment there, wouldn't it?