how can I make my blackberry wine less bitter?
Sweeten it. Treat the wine with sulfite and potassium sorbate and add sweetener IE sugar or honey dissolved in boiling water which has been cooled. Alternatively you can use a product called "wine conditioner" which is a very concentrated sugar syrup with sorbate already in it. You will still need to treat the wine with sulfite or the sorbate will cloud the wine and take several months to clear.