In what way is the mulch 'wrong'?

Blackberries aren't really bushes as such. They grow on canes and are usually grown on a trellis or wire fence. In the autumn after the berries have been picked you cut down the canes which fruited this year and train in the new canes growing up from the base of the plant.

Your blackberries may be one of the "erect" varieties. If no trellis was used on them in the past, none may be needed. Some years a heavy fruit set may cause them to lean over and get in the way. Driving a single metal T-post into the ground near each clump would allow you to lasso them near the post so you can more easily mow around them.

We used a double galv wire trellis @ 36" and 57". On (2) tee posts anchored to deadmen in ground. (a 2'-0 galv rod with a loop on one end)

trellis is approx 18'-0 post to post. I trimmed most of them as they reached the second wire. very sturdy.

We might go taller next spring. the laterals produced after trimming grew above 8'-0

and have set fruit.

Mulch is good to break down heavy soils, aged pine bark holds moisture and doesn't affect ph. Why was yours wrong?

you may just have to learn the plants identity & requirements through experience.

still a very decent find indeed! congrats!!