Can black iron pipe be use for natural gas water heater?

only for the gas connections , not for the water connections because if you do hook up black iron pipe for a water connection you could possibly have a little flood years later when you forget you even have a water heater and water will always remind you.

Yes and you can use black pipe for steam / Steam condensate / Oil lines / Fire suppression / Hot water heating (hydronics) air lines / HVAC chiller applications BUT not on fresh water as fresh /potable water contains oxygen and this will corrode the piping and you cannot direct connect any black iron to copper.