Blueberries like acidic soil and soil with a high organic matter, so add peat and possible sulfur to make the soil more acidic. The cherries probably just need more time. As for fertilizer. If you add nitrogen it will promote leaf growth at the expense of fruit so get one that stimulate
root growth, stem vigor, and flower and fruit production. Fertilizers of this sort contain little nitrogen and higher levels of phosphorus and potassium; the N-P-K ratio may be 3-20-20, for example. And make sure that the nitrogen is a acidic type vs the usually non-acidic ones, ones that are usually formulated for citrus
trees and acid-loving plants such as blueberry, camellia and rhododendron.
I however think that the reason why you are having such a low fruit output is because you have so many plants in such a small area.
8ft is realy only good for 1 cherry tree that yields 40lbs after 6 or so years. so if in that same 8ft you are now getting 3cherry tress that "only" yield 20lbs each for a total of 60lbs vs 40lbs you are doing good. Not to mention that you are also getting a few blueberry and maybe some herbs and bush beans too.
Iterations are fine, we don't have to be perfect
My 2nd Location:Florida HardinessZone:10 AHS:10 GDD:8500 Rainfall:2in/mth winter, 8in/mth summer, Soil:Sand pH8 Flat