Personally I never bother blanching cucumber. I find that Otos and Ancistrus eat it fine without.
TOMATOES! Really?! I'll have to try that...you know what I was amazed by was tomatos! My ottos love em! I have three that engorged themselves and dusted a large slice in 24 hrs. I did blanch my cucumber but I guess I won't from now on- now I'll prob try to freeze em actually