When the system itself is tilted against blacks or other minorities, or when it looks the other way in cases of private, individual racism.
A few easy examples:
Gerrymandering This is a tactic that both parties know how to use to the fullest, despite the Democratic Party claiming the high ground on the issue. In the South gerrymandering is frequently used in a racially-based manner. There is a science to how to do it effectively, but both parties are
very good at it. However, there do tend to be more leaders on the Democratic side calling for non-partisan commissions to draw the voting district lines.
Reducing the number of polling places
Texas closes hundreds of polling sites, making it harder for minorities to vote Here is one of many examples of a Republican-controlled government simply reducing the number of places to vote, with most of the negative impact being in black and Latino neighborhoods
Dodge City, Kansas
Dodge City polling place debacle: voter suppression or incompetence?
Dodge City has only a single polling place for a city of 13,000. They moved it from the center of town to a place outside of town, making it necessary for most voters to drive or take a bus to vote. This was a greater hardship for Latinos in the city, who make up 60% of the population.
Eagles Landing, Georgia
Stockbridge de-annexation, pro-Eagle’s Landing bill heads to governor
This seems to be a mostly neutral news account, but the issue was that in the black-majority city of Stockbridge, a group of suburban residents floated a petition to de-annex a portion of the city from Stockbridge and form a new city, Eagles Landing. Problem is, the suburbanites were mostly white, and they wanted to take away a big chunk of the commercial part of Stockbridge, which would have been economically devastating to Stockbridge. This was a classic case of take-from-the-poor-and-give-to-the-rich. Fortunately, the voters defeated the measure.
There are many, many other examples.