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i don't know your area, but i'm from western PA and i know that too much water, too little water, too much sun and or heat are huge culprits. They tend to grow well along woods where they have sheltered sunlight, but not too hot. Do you have different berries (rasberries in particular) or a black walnut tree close? These things don't mix real well with blackberries. This has been my experience and i've grown berries my whole life. Also pruning them down will be a benefit. I hope this helps

They can have an off year, but it could also be that they were pruned incorrectly, which removes the fruiting wood, or they may have contracted a virus that rendered them sterile. This article may be helpful:
https://www.gardeningknowhow.com/edible/fruits/blackberries/blackberries-not-fruiting.htm

If you are looking for a chemical solution, a broad leaf specific herbicide combined with a grass specific herbicide will take care of most weeds but will not hurt the blackberries.

For a more organic solution, a thick layer of mulch laid down (at least 3") will help suppress most weeds in the patch.

If they are truly maggots, I would avoid eating them as they tend to be small and plentiful, which means they are hard to get rid of. If it is just a few caterpillars, soaking the berries in a bowl of water for a 1/2 hour or so will cause the caterpillars to rise to the top.

My guess would be that you may simply be picking them too early. Most people tend to pick blackberries when they are a nice glossy, dark color when in fact you should wait for them to dull in color before harvesting.