At 232' F. enough water has boiled from the syrup, and the sugar is concentrated enough to give you a rich syrup. But cooking to this temp can be challenging as it is easy to scorch the food solids in the syrup. An alternative way of making your blueberry syrup thicker is to add more granulated white sugar, to increase the ratio of syrup to water. This may make the syrup too sweet though.
Thickeners that will work include corn starch, arrow root, fruit pectin, and tapioca starch. Gums will thicken but if used alone may give an unwanted stringy texture to the syrup.
When using starches, remember to make a slurry of the starch with a bit of water, to form a medium thin paste. Slowly pour the slurry into the boiling mixture. And know that the texture will be different when the syrup cools. And starch based thickeners can leave things a bit clumpy. This is not necessarily a bad thing. Just be aware that it is possible.
You have one more alternative that will leave your syrup silky smooth, and not overly sweet. Add corn syrup to your blueberry syrup. This will thicken it without adversly changeing the texture or flavor.
Seeeeeeya; Goodweed of the North
Seeeeeeya; Goodweed of the North