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