I think you need to start from the realization that offender statistics are systemically biased and recognize that your certainty level here should be capped.

Your analysis here is based on another fallacy, which is that there's a direct connection between the crime of homicide and capital punishment. In fact, no: only a subset of murders, aggravated by circumstances mostly not present in gang shootings, attract death sentences. Further, some presumably large fraction of the black/black murders you're considering occur in jurisdictions that are statistically (or, as in the case of Chicago, legally) unlikely to apply capital punishment.

To reach a conclusion that there aren't racial disparities in capital punishment, you have to de-confound the homicide statistics to eliminate murders that are effectively or statistically ineligible for the death penalty, and then make comparisons.