Nami's Tidecaller's Blessing - Overwrites all other on-hits?

So just now I was playing a game and noticed that anytime someone had my Tidecaller's Blessing buff on them, their Black Cleaver wasn't applying any onhit effects, and they weren't getting any lifesteal either.

Is this working as intended?

I went ahead and bought a Black Cleaver AND a vamp scepter at the end of the match to test it myself, to make sure, and got the same results!!!!!!