I'm originally from Texas, where poblanos are commonly found in grocery stores. I moved to California a while back, and here they're still pretty easy to find, but they're always labeled "pasilla". Even at a local produce market with a decent amount of Mexican food (and Spanish-speaking employees) - and just recently I noticed they even have anchos (dried poblanos) labeled as "dried pasillas". I did manage to find one local Mexican restaurant menu online describing a chile relleno as a stuffed poblano, but otherwise it seems pretty universal.
So where did this come from? Via the above-linked wikipedia article, I did find a website saying it's the fault of the California produce industry for labeling them this way, but that's not the original reason. Is it all just the result of a single person getting it wrong a long time ago, and an entire produce industry building off of that?