How important is it, for democracy, that black people can vote?
This was a big step forward for democracy. In the USA's South during about 1900-1970, blacks were almost wholly disenfranchised by "Jim Crow." This has led some authors to claim that the USA was not a democracy before the 1970s.
But let's make a more quantitative assessment so that we can compare with, e.g, the importance of women's suffrage. How many election results have actually been directly changed by black votes?
Women since 1980 have voted more Democratic than men (the "gender gap") with, e.g. 54/43% of women voting Gore/Bush in 2000 versus 42/53% of men. (CNN's Voter News Service exit polls.) We can directly compare women with blacks because blacks also favored Gore by 90-10 over Bush (versus 42-54 among whites).
In 2004 both the race and gender gaps were somewhat smaller but still there and still pointing in the same directions. CNN's exit polls said men favored Bush/Kerry by 55/44% while for women it was 48/51%. Whites favored Bush/Kerry by 58/41% while for African-Americans it was the opposite: 11/88%. (The overall count was Bush 51%, Kerry 48%.)
Let's compare the effects of Blacks and Women in these two elections combined. Women were 54% of the voters and differed from the mean by 4.8% yielding net perturbing effect (the product of these two numbers) 2.6%. Blacks were 11% of the voters and differed from the mean by 41% yielding net effect 4.5%. So even though blacks are far fewer than women in numbers, they currently actually have a greater vote-shifting effect. In that sense, enfranchising blacks was actually a greater step forward for US democracy, than enfranchising women.