Blackberries are considered a weed in NW Wash. If they are happy they will grow like crazy. I think that eye screws is one of the best ideas I have ever heard for stem containment.
You do need to prune the bushes back, or they will attack you, and it is NOT pleasant, and it can be hard to get loose. Depending on how long you have Indian Summer ours go until the first hard frost, so that is a lot of growing time.
Mine grow right over the property line (like 20 ft away), and I have to race the critters to get them. All of my berry picking is out the back door or in memory. My sister has a few places in the neighborhood where she can get a couple of dishes for breakfast, and if we're lucky and she gets out early a jar or 2 of jam.
There is nothing like your own fresh berries. Have fun!