Personally, I'd check at night with a flashlight to see whether you have snails or slugs feasting on the blackberry leaves. If so, a soap spray won't do any good, b/c snails and slugs are not
insects; they're gastropods. The most effective control is hand-picking, followed by...ah...feeding to chickens or ducks, if you have them (or friends do), or "recycling" them into the ground.
Ascertain the culprit before treating; you may be "barking up the wrong tree," if I may mix metaphors?
Sunset Zone 17, USDA Zone 9