Black chia seeds come from the Salvia hispanica, which is in the same plant family as mint. What makes them so unique among seeds and nuts is their high Omega 3 fatty acid content - 1,900 mg per tablespoon.
The amount of antioxidants found in chia seeds is high, but what's more impressive is that with the whole seeds (non-milled) you can almost double the ORAC value simply by soaking them overnight. Why does that occur? Because when you soak them, it initiates the early stages of sprouting, and that creates more bioavailable nutrients.
The same plant, when a recessive gene is present, produces white chia seeds but their ORAC value is significantly lower, albeit still relatively high when compared to the vast majority of foods.
Research Support, Non-U.S. Gov't: Brunswick Laboratories For Omega 3 Chia LLC Report (PDF) Oct 2008