Blackboard Inc.’s pending acquisition of rival WebCT Inc. would merge the two largest makers of educational software, leaving a competitive void and potentially sparking antirust concerns.

The proposed $180 million cash deal would give the merged company a near monopoly, given Washington-based Blackboard controls roughly 45 to 50 percent of the U.S. market for software used to conduct college and other school classes, among other functions, over the Internet, while WebCT, of Lynnfield, Mass., holds about 35 to 40 percent, according to Eduventures Inc. “Anyway you cut it, you’re talking about 65 percent to 75 percent of all U.S. institutions,” said Catherine Burdt, an analyst with the Boston educational research firm.