One way Black voters were prevented from voting was the POll Tax, which denied votes to people who payed no income tax (as many black people had no income). The most famous way black voters were kept from voting in some parts of the south, were laws mandating that all voters must pass a reading test prior to voting. White voters would rarely, in fact, almost never, be challenged at the polls to take these tests. Only black people were given them. The Black voters would arrive and be given a book to read a passage from. The book would be written in a foreign language like Greek or Classical Latin, and of course, inevitably, the black voter could not read it, so they would not be allowed to vote.
Nowadays, however, more modern, computerized ways of voter supression are being used, and almost exclusively by the Republican party. The practice is called Voter Caging, and is traced to the telemarketer term used to describe how mass marketing firms send out junk mail to addresses on their databases, and erase the entries of people whose mail comes back as "undeliverable".
This is exactly what happened in the 2004 election. But the story is a little more complicated than that. Republicans have been challenging black people's votes, and districts which vote mostly democrat, all throughout the 20th century. They in fact, are the only political party that regularly makes accusations of voting fraud, and which seeks court orders challenging whole districts and individuals.
According to BBC reporter Greg Palast, best-selling journalist, and accused terrorist, the GOP has offices all over the country which work in advance of elections to challenge votes in every state.
According to Slate magazine:
The Bush-Cheney operatives sent hundreds of thousands of letters marked "Do not forward" to voters' homes. Letters returned ("caged") were used as evidence to block these voters' right to cast a ballot on grounds they were registered at phony addresses. Who were the evil fakers? Homeless men, students on vacation and—you got to love this—American soldiers. Oh yeah: most of them are Black voters.
Why weren't these African-American voters home when the Republican letters arrived? The homeless men were on park benches, the students were on vacation—and the soldiers were overseas.
In 1986, the RNC tried to have 31,000 voters, most of them black, removed from the rolls in Louisiana when a party mailer was returned. The consent decrees that resulted prohibited the party from engaging in anti-fraud initiatives that target minorities or conduct mail campaigns to 'compile voter challenge lists.'
Back in 2004, many of the Republican's challenges were routinely denied action by the courts, which had come to see the annual voter challenges for what they were.
New targets for the Republicans now include people whose homes were foreclosed on during the recent mortgage crisis. Apparently, if you've lost your house, you stand a good chance of losing your vote, as well.
The whole process by which the Republicans perform this voter supression, is outlined in this report, which explains the whole chain of discoveries that led reporters to figure this all out.
But perhaps the biggest eye-opener in this is the fact that many of the Judges that the Bush administration fired amid great contraversy, were judges who declined to investigate Republican challenges to votes during the 2004 elections. Apparently the republicans are punishing them for not helping them prevent more people from voting against them.