U.S. boardrooms are not the only setting that remains largely white and male, which could limit the talent pool recruiters might draw from. While sitting CEOs are a popular choice for boards, among the top 200 companies, only four, or two percent, were led by blacks, according to the Spencer Stuart data.

Among partners at big U.S. law firms, only 1.8 percent are black, according to a study released last month by the National Association for Law Placement. And just 5.9 percent of U.S. college presidents were African American in 2011, the most recent year studied by the American Council on Education.

Some counter there is still plenty of talent available.

"The pool of qualified African-American candidates for U.S. boardrooms is still quite deep and should not be a limit," said Kurt Schmoke, an African-American who is president of the University of Baltimore and a director of Legg Mason Inc and McGraw Hill Financial.

Schmoke said he has urged executive search firms to add minorities to their databases. "Sometimes you have to remind them there are other people out there."

Indeed, some big companies may be casting their net more broadly than they have in the past.

Technology leaders Apple Inc., HP Inc. and Hewlett Packard Enterprise have each named new black directors recently.

None are currently CEOs at other big companies, a traditional criteria for board recruiters that may have limited the number of minority board candidates available, said Mary Bush, an African-American director at companies including Discover Financial Services and T. Rowe Price Group.

Such changes may eventually improve the representation of blacks and other minorities in boardrooms -- if board leaders stay focused on the topic, Bush said. "When you are coming from behind, which we were, there's definitely a long way to go," she said.

However, there is some evidence that big activist funds tend to run slates of mainly white men when they are fighting for board seats. Among the ten campaigns against the largest companies that have gone to a shareholder vote since 2013 identified by financial services group Houlihan Lokey, activists nominated 50 directors for board seats. Of that group, about 40 were white men and 6 were white women, according to a Reuters analysis of securities filings, company websites, and checks with individuals.

Among the nominees was just one African-American, Lionel Nowell, part of the Starboard Value slate that won seats at Darden Restaurants last year. Nowell, now a Darden director and a former PepsiCo Inc executive, is also a director at Bank of America Corp, where a representative said he did not wish to comment for this article.

Representatives for Starboard Value did not respond to questions about its slate, which also accounted for four of the six women in the group.

Among other big activists that brought the proxy contests, representatives for Trian Fund Management and Jana Partners declined to comment.

Representatives for Elliott Management did not return messages Mitch Ackles, president of the Hedge Fund Association trade group, said activist funds may be more focused on shaking up corporate boards to improve shareholder returns than on diversity, perhaps because the hedge fund industry is largely white and male.