Yes he did. Volume 38, end of the volume. Volume 47, in the Magic Show file, when Kazuha grabs his arm.
When he is blushing when he is talking about Kudo it's because he's exciting about the fact he will be able to solve a plot or another mystery or talk about it.
I've never seen any case of Heiji blushing for Kudo's HIMSELF.
Anyway, you may love Boys Love Couplings. That's your choice. But you can't find any evidence of "Heiji's love interest" for Kudo.
But because I'm writing a big file about Hattori and romance, if you have some examples of fan service scenes between Kudo and Heiji, I would be glad you could tell me them. I noticed a lot of them but I'm not a BL fan so I may have missed some. Thanks for your help.