Blacks have extremely low visual memory so it is very difficult for them to recall digits in reverse.

Race and intelligence (explanations)#Spearman's hypothesis

IQ tests contain one or more sets of test questions of different varieties. Individually administered tests often are composed of subtests that have different homogeneous item contents. The mean Black-White difference varies considerably across tests with different contents. For example, the BW gap is larger on tests that require the recall of a series of digits in reverse order than on tests that require the recall of a series of digits in forward order. Across a large number of test, the standardized mean Black-White gap varies from near zero to over one standard deviation. According to , "this variation between tests in the size of the standardized mean W-B difference is not explainable in terms of test bias or in terms of differences in types of item content or other formal or superficial characteristics of the tests."

"The g-based factor hierarchy is the most widely accepted current view of the structure of abilities" Correlations between an IQ subtest's g-loading, and the magnitude of the Black-White-East Asian score gap for that subtest (;  pp. 47-8;  pp. 138-9).  For example, the Black-White gap is greater on backward digits span (a test where subjects repeat digits in the reverse order that they are given, and the more g-loaded test) than forward digits span (a test where subjects repeat digits in the same order that they are given, and the less g-loaded test).  As predicted by Spearman's hypothesis, the B-W gap is largest on the most g-loaded tests.  Narrowing of the B-W gap has been seen mostly on less g-loaded tests, such as literacy tests. Murray, C.  (2005) "The Inequality Taboo".  Commentary Magazine, September 2005. Arthur Jensen argues that there is no independent evidence that the environmental explanations commonly given for the Black-White gap (for example, "past history of slavery", discrimination, "caste status", "peer pressure against 'acting white'", etc) have an effect on g (, p. 510).
|The author of multiple factor analysis, L. L. Thurstone (1947), warned "we must guard against the simple, but common, error of merely taking a first centroid factor, a first principal component, or other mean factor, in a test battery and then calling it a general factor".  "Spearman's g, then, is a "statistical artifact." (C. Loring Brace, 2001)

The gap is larger to recall digits in reverse because blacks are much lower in visual intelligence.

"a test where subjects repeat digits in the reverse order that they are given, and the more g-loaded test"

IQ tests are biased against blacks because they are designed to test people with high visualspatial intelligence.

This is obviously original research, but I cannot find no sources that supports my argument (even though I am not black).