The g factor refers to general cognitive abilities; things that make you better or worse at a wide variety of cognitive tasks. Differences in the g factor are partly due to genetics, with estimates for its heritability ranging from 20% to 80%. Brain size has been implicated in g, IIRC accounting for like 10% of g.

There are various studious that investigate whether there are genetic differences in g between black people and white people. A lot of them are quite questionable because they are very indirect, looking at e.g. whether phenotypic differences after controlling for various correlated social factors.

In principle the most direct approach would be to just have a good genetic model of differences in g and applying that to genetic data that distinguishes the races. However, genetic models of psychological traits are still in their infancy, and we do not understand the mechanisms of genetic factors in much detail beyond their statistical average causal effects. This becomes a problem for various reasons that have been discussed in various places and which I won't go into in detail here.

Over time, presumably we will develop better models, to the point where eventually we will have a good mechanistic understanding of how g develops. At that point, presumably someone will do a definitive, solid study of the race differences in genetic g.

 (white genetic g - black genetic g). To quantify genetic g, we will measure it with the within-race standard deviation of phenotypic g being 15. Mechanisms such as "black people have darker skin and society discriminates against darker-skinned people, giving them a lower phenotypic g" must be controlled for by the study. Mechanisms such as "black people know other black people and g is socially contagious, bringing the black g down due to historical disadvantage" or "black people have socioeconomic disadvantage in their families" may not confound the study in order for it to count for this market. 