You are on very close to completing the equation; So if 14-15% of the population is black, and the DNC locks down 90% plus of the black vote... the black population is amplified to reflect about a 38-45% voting bloc of black voters.
Now because of free college, more black women attend college and complete more than their black male counterparts or in the rest of the student loan or consumer purchase college attendance. College tends to expose children finding themselves away from home for the first time being woke to communism under the guise of socialism, exposure to a DNC village, and then the gay and lesbian facility the preys on the young.
Once a population has been properly socialized or bullied into a set of values, e.g., their adult mentors who are communists and homosexuals, as a normal formative experience with a radicalization to ensure foot soldiers are created for the Marxist Long March in America... our youth come out of college with student loans (even the ones on full ride scholarships) because the fun never ends.
The DNC grooms a population ready for student loan relief, hypersexualized and homosexualized by association or participation, and political indoctrination.
Woke and released like fish in a hookless live catch pond.... they head back into the greater society carrying their social indoctrination baggage. Every college kid is tolerant of Abortion because they have experienced it as a common place expediency in their value driven group; homosexuality, check; Marxism, check; Environmentalism without an learning experience of cause and effect, check.
This is why the DNC is so lesbian, so black, so homosexual, etc.
The intolerance was taught in College, where there is no debate or discussion. Only Shunning, imagine that. Conservatives are accused of false crimes and teachers throw them out of class or withhold grades.
It doesn’t take a brain child to attend college... it takes a paying customer in light of reform in the marketplace of college and government encouragement.
The DNC needs Marxist foot soldiers, and women make the feeling radicalized voter component that the Old Lesbian Hillary appeals.
The voter bloc is locked down - 1/3 DNC, 1/3 RNC, 1/3+ Independent - so that your 12.6% or my calculation of 13-14% of the population being black has magnified consequences for the DNC - there voter base if black, college educated and hypersexualized and it represents nearly 50% of the total DNC core in the primaries.
I'm not sure you can assume a normal distribution for intelligence. The median will be at 50%, but if the distribution is skewed by the existence of profoundly stupid people, then it's entirely possible that more than 50% of the population has below average intelligence.
As evidence of this possibility, I submit the last Democratic debate and AOC who by themselves probably lower the mean intelligence of the US at least 1% below the median.