Large berries form on healthy plants that have plenty of water and fertilizer. Use a slow release plant food like Osmocote on your berries in late spring and mulch the roots with a compost or wood chip mulch. The pink spots could be the start of rust,a fungal infection common on blackberries or it could be single kernel in the berry that did not form properly due to a number of small plant stresses or due to poor pollination. Keep the plants  away from shade and root competition for the best harvest. Keep growing, Marianne Binetti