Blackberries not developing

I do know what you are talking about and have seen it myself!  I think the problem is due to inadequate pollination. The drupelets (your blobs!) only develop when the ovules are fertilised by bees.  We all know that there has been a problem with the decline in bees and wet springs & summers or even hot days (what are they!) can reduce the number of bee visits to the flower and hence reducing the quality of the fruit. 

Incomplete drupelet development can also be a symptom of exhausted reserves in the plant's roots, or a viral infection such as Raspberry bushy dwarf virus but I doubt this is the case as you do not have reduced yield and your fruit are not crumbly.

Annual pruning is beneficial too, in Stephanie's Kitchen Garden I pruned and tied in the horizontal branches in late autumn and we have many more flowers (and hence fruit) than last year.

You might be interested in our Beepol Bumblebee Lodge this has become very popular with customers wanting to increase the bee population in their gardens and is quite fascinating to watch.

Kind Regards

Lynn Burton
Horticultural Advisor / Kitchen Gardener