Why would sulfuric acid turn gray/black?
The only reason I can think of that would cause the solutions would blacken over time is if the solutions reacted with organic substances. Sulfuric acid is an extremely strong oxidizing agent, and dehydrates many organic compounds to produce carbon in the form of graphite, and water. For example, the reaction of sulfuric acid with paper (cellulose) is:

$$(\ce{C6H10O5})_{n}\ce{(s)} \xrightarrow{\ce{H2SO4}} {6n~\ce{C(s)} + 5n~\ce{H2O(l)}}$$

and can be seen in the following photo:



If a considerable quantity of organic material was reacted in your solutions, the $[\ce{H+}]$ in your solutions would actually decrease, so I would recommend titrating one of your solutions to see if this has changed since you bought them.