Unless you found your potting soil in a third world country, there is no magnesium deficiency. Don't add Epsom salt.
You could bury the entire blackberry plant and it would root all over the stem and pop up in more places.
Those are stress colors but, I suspect this stress is caused by too much sun and lack of water. Blackberry bushes can handle too much sun if they have enough water. The newest leaf looks better. Is it shaded? Are you keepig the soil moist?
Wild blackberry bushes are always found along creeks, and rivers and near ponds or growing on seeps. They like water.