Graininess caused by excess undissolved sugar is fairly obvious. The grains will be sweet and will dissolve on the tongue.
Alternatively, with some fruit including blueberries, the skins of the fruit can be dry or tough and stay in grainy fragments in the jam. Again, this is obvious. The individual shreds will be dark and flat, etc.
Another, in my opinion, more subtle and more likely is that the graininess was caused by excess pectin gelling.
When pectins overgel they pull tight and form small, stiff granules. These are bland and rubbery. Very unlike sugar or fruit particles.
Normal, thermally reversible, pectin does not often over gel. If it does, reheating and possibly adding a little liquid will fix the problem.
LM pectins, on the other hand, can over gel fairly easily and as they are not thermally reversible it is impossible to fix the jam when it happens.
See this article about pectin, in particular the 5th page about LM pectins.
Blueberries are very high in natural pectin. Blueberry puree will sometimes set up as jam without any added sugar or pectin. This abundance of pectin may have caused the added pectin to over gel and form the graininess that you saw- especially if you were using low sugar pectin in your jam recipe.