I've had some subtle blackberry beers that were quite OK, as long as the blackberries are not too pronounced, and IMO they can work well in sours with diligent and restrained use.
On the other side of the scale, I had a Blackberry Steel Reserve last week, and it was way way way too sweet and overbearing. This means it was heavily back sweetened and either completely filtered from yeast, sorbated, and/or pasteurized. It also didn't taste like beer at all, not even like Steel Reserve, more like soda with a resulting buzz if one could consume enough of it, which I doubt would be possible.
That Blackberry SR reminded me of today's Lindeman's Framboise (not what they brewed 40-50 years ago), all the sweetness, but without the complex Lambic backbone character, desired tartness, and beautiful Raspberry flavor and aroma. Again, closer to a grape soda, and not very palatable as such.