Few things will create a ...

Few things will create a black color and typically most will be related to anaerobic growth and all are not good. 

If the black is coming from most iron salts, collect some of the material in a test tube and insert a small magnet nearly all black iron salts are magnetic. So if you get a build up of crud on the magnet, Some of it is going to be iron.

You can also elevate the pH to over 8.2 (very small scoop of baking soda) then add some bleach if it goes red. It is iron.

I'd start to look at any mild steel water on the shell H/E's  that might be running hot.  It is not uncommon in a system as you have to pull in lots of bacterial food and the bacteria setup growth in the slow/low flow coolers and then you get a massive release of all that crud.

If it is iron, you have a severe corrosion problem and you need to address that quickly.

Hope this helps you.