Go heavy with cow manure and sand if go real heavy you have rise in sand ph. I stay away from moss not needed. A liquid iron in small amount be needed. A lot cow it hold water better its mostly organ matter fertilizer with 10-10-10 complete fertilizer has all metals needed for plants. Blackberry plants heavy feeder.
Cow manure low in nitrogen, phosphorus, and potash 1-1-1 so use a lot your sand color should be soil black that way look as good soil for blackberries black sand. Chicken manure 3-3-3 make even better mix with sand fresh is best chick are cow if smell nasty its good if no smell its organic matter no value feed berry plants.

That's right, don't use the peat moss. The ashes won't help much, either.

Sandy soil might be hard to keep watered properly. See if you can get ahold of some ordinary soil, that should be half of your mixture.

Gator Rider, thanks for your follow up. I had a really hard time understanding some of the sentences. Is there anyone out there who can please help by clarifying?

Also, general question to everyone: How far down do I have to dig? To replace the builder's sand with the soil mixture for the blackberries?? PLEASE, if someone could help, I'd appreciate it.