Here's how the black vote has trended... according to this both times Reagan ran he made huge losses in the black vote, but since then, every GOP incumbent has made gains while every Democratic incumbent has made small losses. Although it looks like overall, ever since Reagan the Republican candidate has made gains among the black vote in every election except for 2000 and 2008
Carter
1976 D+66
1980 D+69
Gain of 3 (+16 compared to overall electorate)
Reagan
1980 D+69
1984 D+82
Loss of 13 (-21 compared to overall electorate)
Bush 41
1988 D+78
1992 D+73
Gain of 5 (+18 compared to overall electorate)
Loss of 4
Clinton
1992 D+73
1996 D+72
Loss of 1 (-3 compared to overall electorate)
Bush 43
2000 D+81
2004 D+77
Gain of 4 (+1 compared to overall electorate)
Obama:
2008 D+91
2012 D+87
Loss of 4 (-1 compared to overall electorate)
Trump:
2016 D+80
2020 D+75
Gained of 5 (+9 compared to overall electorate)
I doubt reagan made big losses with the black voter, he improved in the black belt and most big cities.