Blackberries are one of those things that are difficult to transplant along with dewberries. I would suggest wsing them and then about 1st to 2nd set of true leaves transplant. Granted they are not like a tree but when planting they are as picky as a tree.

Meaning that how deep they were when they germinated is how deep they need to be when they are planted. No more no less.

If that sounds difficult it might be best to direct sow them but I would protect the critters from eating them that is for sure. Maybe a short term cover, but with plenty of ventilation.