I can understand why you might have a passion for Marion blackberries! Can they be grown to produce fruit in the Hermiston area? The short answer is yes! However, it will take some effort.

There are many microclimates in your area -- the warmer the microclimate the better. The biggest challenge to growing Marion outside the Willamette Valley is that this cultivar (variety) of blackberry is sensitive to winter cold and to summer heat -- both can be challenges in your region.

Marion will suffer damage from winter cold when temperatures in mid-winter drop to less than 8F. Trouble is, cold weather in late fall/early winter or late winter/early spring of temperatures in the low 20sF can cause damage. I'm attaching a picture of what cold damaged Marion looks like in spring. Buds will not break. Ones lower on the cane, from more mature or protected canes will flower. If the entire cane(s) are damaged then the plant will still survive. New canes will grow and then will produce fruit next year (assuming there is no cold damage next winter). The best way to try to protect canes from cold is to leave the primocanes (first year canes) on the ground most of the winter and then train them up onto the wire in late February or early March before bud break (see picture with canes on ground before training in foreground and a trained plant in the background). If it gets really cold in your area you can protect these canes on the ground by covering them with a mulch -- however, if it rains a lot the canes will rot in winter. If it is forecast to get cold in late winter/early spring and you've already trained the canes, then you can cover them with a spunbound rowcover (will give you a few degrees of protection).

If you've succeeded in getting the canes through the winter and you've got good flowering and fruiting, then you have to hope that you don't get heat damage where the fruit just "jams" on the plant (see picture) or u.v. sun damage  where sections of the berries turn white. These are hard to avoid and are typical in warmer regions with low humidity (we even see these in the Willamette Valley when temperatures get suddenly hot). If you see this a lot, then you may consider covering the plants with shadecloth during fruiting.

Please find more information about growing blackberries in your home garden in my publication available via the OSU Extension Service: http://ir.library.oregonstate.edu/xmlui/bitstream/handle/1957/18929/ec1303.pdf

I know this seems like a lot of work to try to get fruit on Marion......but if you are successful, it will all be worth it :-)