Man with that topic title you had me worried for a minute. Blueberries grow in summer. You can forage for them or you can grow them yourself, but you will have to wait till summer. For lemons you need a lemon tree or you can buy them directly from Remi. I’m assuming you are on version 1.3 where Remi actually exists in the game.
She should come to town on Mondays (when you have a crescent moon symbol near the clock) and Thursdays (when you have a squiggle that looks sort of like a 4 near the clock). Assuming no odd bugs are happening she should set up shop outside the general store probably from around 9ish to 5ish. She will sell you saplings, one of which is a lemon sapling. Or as I just remembered she also sells the fruit directly, but its fairly pricey. Oh and one other note, she doesn’t come to town when it’s raining currently, so if it happens to rain on those days you’ll be stuck for now. I plan to have something to mitigate that eventually.