This is pure conjecture on my part, but I will add it to the discussion - - - I have heard it said that the less golden-yellow the pollen is, the lower the fertility.
In my garden, Randy Stephens, Bubba on Tour, and Leslie Renee are examples of those having golden-yellow pollen and all are quite fertile. Alan Lane Agin has pollen that is much less golden, but is still fertile. I have one seedling that has gray "pollen" while its siblings have golden-yellow pollen. I tried a few crosses with the gray pollen and had none take, not even an aborted pod, so I presume it is infertile. I also have one cultivar in particular, Blissful Blackberry Pie, that has lemon-yellow pollen that is, as you describe, lumpy and hard to use. Further, it seems to have low fertiliey based on the percentge of successful crosses. I freeze pollen for future use and see some (sorry, can't remember which, but both cultivars and seedlings) come out of the freezer vary hard and lumpy and difficult to use while most are still fluffy, so this seems to be dependent on the particular plant. This same plant, Blissful Blackberry Pie, also often fails to produce a viable pistil when the temperature gets hot.
Larry
