So, I see two options for Izumi:
1) She genuinely thinks her feelings for Fujishiro are just friendship and applies that logic to Fujishiro's feelings for Kurokawa (cope route)
2) She knows and tries to stop Fujishiro from realizing (either because she's jealous or because she doesn't want her to get hurt) (gaslight route)
With the way the manga's been portraying Izumi so far, I'd go for the second option, but I guess we'll see soon enough