It could be leaf miner insect damage. There are many leaf miners infest blackberry and raspberry like Metallus rubi, Metallus pumilus etc.,
Generally the adult females lay eggs on the leaf surface. The hatched larvae feeds on leaf mesophyll tissue by leaving the upper and lower layer intact (called mining) which results in the appearance of conspicuous blotches (image attached). The mature larvae drop out from the mine and pupate in soil.
To control the insect you can squeeze the leaf at the blotch area between two fingers to crush the larvae or remove the infected leaves and destroy them. Use yellow sticky traps to catch adults. Mulching also helps in reducing leaf miner population.
Mine of Metallus pumilus on blackberry
Blackberry leaf miner larvae
Mine of Metallus pumilus on blackberry
The blackberry leaf miner (Metallus rubi)
Mine of Metallus pumilus on blackberry
Fall time. over 7 years ago.