I did some looking to try to sort it out.
In 1923, when Jack Trice played,
ISU was in the MVIAA (Missouri Valley Intercollegiate Athletic Association) with Drake, KSU, UNL, OU,
KU, Mizzou, Washington U in St. Louis, and Grinnell from what I could gather. In 1928 the big schools formed the conference informally known as the Big 6 and the small schools formed the Missouri Valley although both apparently used the same name and claimed to be the rightful heir of hte original 1907 conference. CU made it the Big 7 in 1948 and OSU made it the Big 8 in 1958. It was officially named the Big 8 in 1964.
The article on Robinson says there were no other black players in the conference at that time which could be correct but it still seems like the color barrier for the conference had already been broken by Jack Trice, if not someone else before him.