Sounds similar to compacted, anaerobic, alkaline clay. Maybe a hardpan. Maybe it's stuff the builder brought in years ago. Maybe it's where someone had a bon fire. But dull colors in soil indicates a lack of oxygen.

Mackel

Hardpan can be formed in soils as the result of Iron Oxides, and orangeish red color, or Calcium Carbonate, a grayish colored soil, but both will impede the flow of water enough that drainage would be an issue.
On occassion I have found clumps of organic matter buried in my soil the color of charcoal, or wood ash, usually the result of some wood getting buried.

So if I break it up and spread it around I should be OK?

Could be where someone dumped kitty litter.

If where this is there is a drainage issue adding more organic matter to that soil will help correct that. If there is no drainage problem breaking it up and spreading it around may be another solution.
Given the potential for Toxoplasmosis why would someone put used kitty litter out in their garden?

As the parasite is only a serious concern to infants and those with severly compromised immunes systems, I would imagine most do not care.

People do all kinds of crazy things.

I've seen gray clay in my area, although most of it is yellow to brown. Very dark gray clay does sound like it's got organic matter in it.

T.C.- Organic matter plus oxygen equals healthy, no? It's not the color of the soil, it's the intensity of the color, that determines remediation possibilities or a shrug of the shoulders...yes?...

Sir Pedant

Look up "gleying" and see if circumstances fit.