A couple of weeks ago I bought a Reuben Blackberry plant and I noticed strange red spots on it. Now the leaves are beginning to turn yellow and dry. I used Natural Peat for this plant (local gardeners recommendation).
Certified GKH Gardening Expert
The simplest explanation for the yellowing leaves is transplant shock, which is very common in the first few months after transplanting:
The red spots could be caused by Blackberry Purple Blotch, a disease caused by Septoria fungi, especially if the spots are/were present on the stems as well:
Alternatively, the symptoms for Anthracnose might also match for the spots: