I believe there is no way "terroir" could not be affecting honey flavor and aroma, to use the double negative.
I don't think anyone doubts that honey from different regions will have different flavor - the obvious example being wildflower. Here in Florida wildflower is going to be some mix of citrus, Brazilian pepper bush, palmetto and such, and Canadian wildflower is going to probably be heavy on clover. They'll certainly be different, and they will vary from year to year.
I guess we need to define "Terroir" a bit.
In my interpretation (and I'll be first to admit a humble one) of Terroir what we are looking for is not the difference from place to place, but instead that consistency which could define a place from year to year. In winemaking, each vintage may be different depending on temperature, number of sunny days, rainfall amount, how the canopy is managed, how much fertilizer is used, how ripe when picked, and how selective the sorting among other factors. With all these variables, plus all the cellar operations, and the blending of varieties, you'd think that a wine must be radically different each year.
That's not necessarily the case. With the direction of the winemaker, adjustments are made within the framework of these variables to try to produce some consistency in flavor and style. In the end, to use a gross example (and an expensive one), in the Bordeaux region, a Château Haut-Brion wine tastes different from a Château Mouton-Rothschild, and if you compare several vintages, that distinction remains. There is something either because of or in spite of all those variables that will give a Château Haut-Brion (or any great wine) some distinctive character. In the case of Haut-Brion, even my limited palate can pick it up.
So my question really isn't "will blackberry mead taste different between Florida and Oregon honeys?" I expect the answer to that is yes, though I could be wrong and would like to see it tested as the first part of this. My question is, "do each of these regions have something that makes their blackberry honey (and the mead) distinctive that carries through from year to year?"
If the differences due to weather and blooming of other plants, and beekeeping technique and such, causes each year's honey crop to be so different , then comparing Florida and Oregon blackberry mead over 2 years may be like comparing 4 entirely different meads. Perhaps there won't be something that you can smell and taste that say, "this one's from Oregon." If there is, then that is honey (or mead as it were) terroir.