No, i don’t think he was like that because of fumi but because of the scent of sex on him from danzo
A physical reaction from the scent that caused him to go hard probably, because he and danzo did it almost nightly
and he blushed most likely because he may have been embarassed of how he reacted or how he felt about the situation especially since his brother noticed him get hard, he must have been confused as well since he himself believed and implied that he hated or had held predjudice against danzo and other people
I was slightly confused whether he truly hated danzo or he was attracted to him and just rebelled idk
Hope this helps but this is only my interpretation, u may take this with a grain of salt
But what about that moment that ichi is hugged by fumi (just before pushing him) And he’s blushing and hard like???? Was ichi in love with fumi???? This story was a bit too big brain for me