How to make blackberries sweeter ?

[Plantinyum]

I have two varieties planted together' Karaka Black and Loch Ness. They are with different ripening seasons 'the karaka is doing its thing right now and the loch ness will be shortly after the karaka finishes.
Sooo the karaka fruits are very big and juicy but kinda lack sweetness. Ive found that i need to wait until thefruit drops or is just about to drop and detaches with the slightest of pull, thats when the flavour is most complex and the fruit is the sweetest.
Now how can i make both of the varieties sweeter, sould i stop wattering them while they are ripening? Wright now i am giving them some water every other day, their soil i usually dryish but the plants do not show sighns of water stress?
Thanks, here are a few pics...
I should also add that the plants are still young, its their second year and second fruiting ,so they sould be improving as they age up ?

[Triphal]

Possibly Epsom salt in beginning of spring and end of June supposed to help. Don't forget leave compost, blood meal and bone meal. As you said restrict to much watering during fruiting. This holds good for Jujubes. Most of the ones whom I recommended had luck. But I myself didn't see much of a difference in taste! Last week a friend from Houston called me to thank how sweet his jujubes are this year after following my regimen. Melons supposed to turn sweeter with Epsom salt. But don't forget to provide calcium also as Magnesium retards calcium absorption in plants. Hope this may help. Veseli praznits. Triphal

[Plantinyum]

Possibly Epsom salt in beginning of spring and end of June supposed to help. Don't forget leave compost, blood meal and bone meal. As you said restrict to much watering during fruiting. This holds good for Jujubes. Most of the ones whom I recommended had luck. But I myself didn't see much of a difference in taste! Last week a friend from Houston called me to thank how sweet his jujubes are this year after following my regimen. Melons supposed to turn sweeter with Epsom salt. But don't forget to provide calcium also as Magnesium retards calcium absorption in plants. Hope this may help. Veseli praznits. Triphal

thanks, well i havend feed them all that well now when i think about it, i only added a granular fert thats designed for berry crops twice this year. I have some calcium carbonate so i will add from this , the other ferts ill have to see if i'll find them fere locally...