Not exactly familiar with all that technical jargon, so I don't know what to do
Alright, so it's not hard.
First things first, open data folder in the game folder
Next, find Items.json, it's near the top, above all the maps
Open it in a text editor, like notepad++ (i'd suggest using online json decoder tho) and find ID for the blueberries (ID 37, just to make it easier)
Go to https://www.save-editor.com/tools/rpg_tkool_mz_save.html (because it actually allows adding items into the inventories), upload your save and press Run and then Ok in the pop up
Scroll a bit down till you hit "Party"
And in items add the ID and count how it's shown
After all that, scroll all the way down, press "Edit & Download" and drop the edited file in the save folder in the game folder
Edit: I'd backup your current save in case this method corrupts it (it might happen) Or you fuck something up