Grow your own blueberry bushes and then you can have more blueberries for 3 weeks during the height of summer than you could possibly use for the next 2 years but it's too fucking hot to pick them and the birds eat them all so quickly that you end up having to buy those sweet blue balls from the grocery store for the other 49 weeks of the year regardless.
I'm not a blueberry expert, so I don't know the specifics, but there are compact varieties that are designed for growing in pots. Those would probably do well enough in a window box if you had the right light exposure and climate. Regardless, if you have the right light and climate for blueberries, there are almost certainly plenty of other fruits, herbs, and veggies that would be more productive, easier to care for, and have a higher return on investment.
Blueberries like lots of light, well draining and slightly acidic soil. They are full sun plants. They work great in pine heavy areas, as pine needles add acid to soil. They are generally very robust and easy to grow, requiring only pruning of dead branches about once a year. Most varieties will grow between 4-6 tall, and about the same width. Some go even taller/wider, and make great privacy hedges.
There are "dwarf" varieties, but you are still going to be more like 3-4ft in each direction. They are 100% a bush, so you need space for that. Not very apartment friendly.
Overall they are maybe the easiest fruit plant to grow if you live in the right climate, have light and the space for them. Highly recommended.
Maybe they are talking about the bilberry (also known as "European blueberry" or in Swedish "Blåbär" literally meaning "Blueberry") they are much more compact and the bushes are very tiny and the berries are also much much tastier than American cultivated blueberries.
I grew my own blueberry bush. It took three years to set fruit, that year my pug discovered he could eat all the blueberries off the bush before they were ripe, which delighted him, and then it tumped over and died. A ground cherry is a much hardier fruit but still not pug resistant.