Generally speaking, water is fed to the house on the Cold Water side. It is sent to the Hot water tank, then to the hot water side. I would be checking the pipes that lead to the hot water heater and if there is a way check the hot water side at different places "down stream". If you have Black water going into the tank, then your problem is going to be on the intake side. If the water going directly to the tank is clean, then I would check the tank first. There is suppose to be a pressure release valve on all hot water tanks. You can check the water there by pushing the little lever up. Or if your hot water tank has a spigot on it, you can check it there. If that is clean, then start checking it at the closest place to the tank and work your way out. Ounce you find it, replace the pipes from there and you should be in good shape.

I am not sure of the plumbing laws in Texas, but if they allow Pex to be used instead of copper and steel pipes it will be a lot cheaper for you to play ball. Just for general knowledge, black pipe is suppose to be used as a gas pipe. Galvanized if generally what they use for the water supply, because it will not rust. I would be checking the black pipe that you found for rust on the interior of it and I would try and budget replacing those pipes with galvanized. One last thing, when you start replacing pipe. Put a shut off valve on both the inlet and the outlet sides of the hot water heater. This will help you in the future if you ever have anymore plumbing woes by being able to shut off the water easily to the whole house.

Hope this helps,
Bobo

ps, if you would like to talk to me about this, you can call me at 918-951-9349. You can also text me. Just let me know who it is please.