Black men having bigger dicks has been a pervasive and dangerous myth started during slavery to characterize them as being sex crazed barbaric savages who threaten the purity of white women and deserve to be tamed by men. In the past this has lead to beatings and lynching of black men because white women weaponized this and falsely claimed that black men sexually harassed or abused or raped them.
And though slavery is now in the past (human trafficking and prison labor notwithstanding), this myth still causes harm to the black community, especially the self esteem of black men. When black men are told that all black men have bigger dicks, black men who dont match up tend to try to hide it or keep it secret leading to selection biases which create a feedback loop: black men with small dicks hide it, leading to only black men having visibility in porn and on the internet and in IRL amongst friends and sexual partners, which perpetuates the myth that black men have big dicks which leads to even more black men with small dicks feeling ashamed and hiding it.
Our dicks are one of the most intimate parts of our body and part of our identity and barring surgery and penis exercises which take years to work and are hit and miss and requires too much effort and consistency for most men, our dicks are also one of the most unchangeable parts of us. So this stereotype can have a crushing blow on the self esteem of average or small black men.
And on the flip side, black guys with bigger dicks arent exactly in paradise either. Because of the pervasion of the myth, they end up being objectified and seen nothing more than a big dick and not their body or their personality or their beliefs or what they can bring to a relationship, which tends to lead to unfulfilling and short lasting relationships where girls and guys only want to hook up with them to "experience a BBC" and then leave them hanging in the wind. And dont even get me started on cuckoldry.
TLDR: Black people are people like everybody else and there are big black dicks and small black dicks, the same way it is with any other race or ethnicity. This myth needs to die and people need to let other people be people and not life support machinery for horse sized genitals.