Silvia Saint did have a long term relationship with black porn star, Mr. Marcus.

However, you need to go back even further (the 80s) if you want the first notable IR porn couple. That was between Ray Victory and hot Danish model, Jean Afrique

In fact, Jean ONLY worked with Ray -- creating the super rare IR-exclusive porn starlet: A white girl who ONLY did B/G with a black guy.

Regarding the notion that the Marcus/Saint relationship popularized IR porn sex. That is a bit of a stretch. I'd say IRs "rise" started to happen a few years before that in the late 90s when West Coast Productions started its "Chasing the Big Ones" and "Something about Jack" series. Also, Jake Steed deserves some pioneer credit for his "Little White Chicks: Big Black Monster Dicks" series. Though I don't agree with the theme and monster symbolism, it served as a lightning bolt to fetishize IR sex for white males. If you get the white guys interested, that is where the money flows because they are the majority customers for porn in this country (USA).