Users of the Adblock browser extension may see something today they're not used to: ads.

The ad-blocking giant, which claims to have 50 million users, will still remove advertisements from the Web. But instead of showing the "peaceful, blank spaces you're accustomed to not noticing," Adblock will replace publishers' ads with banners supporting Amnesty International.

The Amnesty ads, which mark March 12 as the "World Day against Cyber Censorship," are a cause that Adblock believes is worthy enough to, well, advertise. The company says the messages—from US whistleblower Edward Snowden, Chinese artist and dissident Ai Weiwei, and Russian punk band Pussy Riot—are a one-day exception to its business as usual.

"Right now, there are billions of people whose access to internet content is restricted and monitored by their own governments," wrote Adblock CEO Gabriel Cubbage in a statement explaining the campaign.

Neither Amnesty International nor Adblock responded to requests for comment from Ars about the campaign.

An important caveat: I installed Adblock on my Chrome browser while writing this story to see what Amnesty's campaign looked like, but I never saw the touted Amnesty ads. My search covered media websites that I read regularly, including the New York Times, Washington Post, and San Francisco Chronicle as well as several that I don't often read, like Engadget and Buzzfeed. I also used Google, Twitter, Amazon, and Facebook. Ads were consistently blocked, but the Amnesty International ads didn't appear.

Ad-blockers turned advertisers

Even though it's for just one day, the move puts Cubbage and his company in the unusual position of becoming a platform for large online banner ads—a business he has denounced vociferously. Even users who let Adblock know they're willing to see "unobtrusive" ads will never see banners the size of the Amnesty campaign. The ads that Adblock deems "unobtrusive" are limited to things like Google text ads and Amazon suggestions. (The companies that have their ads whitelisted also pay Adblock for the privilege.)

The mere possibility that the growing ad-blocking industry could become its own "delete and replace" advertising hasn't gone unnoticed in the marketing and media world, where ad-blocking has become a growing, and controversial, practice.

At a SXSW panel on Friday discussing the growth of ad-blockers, an audience member pointed out the Amnesty campaign to Ben Williams, the head of operations for Adblock Plus (Adblock Plus is a different company than Adblock.)

"It looks like Adblock is actually becoming an ad network without paying fees to the publisher," said the questioner.

"That isn't us," said Williams. "But that possibility is out there. It does blur the line. The name [of this type of software] should really be 'Web customizer,' not ad-block."

Williams then stated a second time, more emphatically, that his company was not involved in the decision to run the Amnesty ads.

He also faced heat from his fellow panelists, which included Forbes' chief product officer, Lewis DVorkin. Forbes is one of a few websites that won't show content to browsers using ad-blockers. DVorkin said they have a 40 percent conversion rate of convincing ad-block users to switch off their ad-blockers.

After Williams described his business model, which charges publishers to be "whitelisted" so their ads still go through to Adblock Plus users with the default options, DVorkin channeled the frustration of publishers.

"I'm not saying this—but others might say—that what you just described is blackmail or extortion," DVorkin said.

"Oh, if I had a nickel for every time I've heard that one," Williams replied. "Clearly, we have found a place in the middle where we provide value to both users and publishers. But we're not the only one." There are more than 200 types of ad-blockers now available, according to Williams, and about 200 million people worldwide use them.

However, Williams added he thinks there's a "ceiling" to ad-blocker usage that's probably been reached. Rates of ad-blocking have hit "26 to 28 percent" in a few high-usage countries, but he doesn't believe it will go much higher than that.

He also emphasized that at his company, the process of deciding which ads can be whitelisted is done in a public form. Adblock Plus has said it will transition that decision-making process to an "independent committee" by the end of this year.

With that kind of user-share, the temptation to use the "peaceful white space" for something, whether it's public service announcements or "whitelisted" advertisements, will be high. As the Amnesty International ads show, one user's annoyance may be another's valued message.