Were your blueberries grainy? I occasionally get a pint that have a grainy texture. I've heard that this means they aren't quite fresh, but they usually taste just fine despite the texture. I'm not sure if this particular graininess translates to a jam though.
Jam can also get grainy from sugar that isn't fully dissolved. This can happen more easily with larger batches, since the fruit can start to boil before the sugar is fully dissolved.
For your particular recipe, perhaps a more thorough stirring after adding the sugar would help. You should be able to turn the heat down once you add the sugar so that you can stir it thoroughly before going back to high heat to boil for that 1 minute.
I live in Italy - the Quattro Stagioni brand here has been around for more than 30 years, and it is considered the golden standard for in-house canning (other Bormioli glass products are well regarded, too); everyone I know who canned something in his or her life have been using them, and I've never heard of anybody saying anything but great stuff about them. It's rare that I go a whole month without eating something that is coming out of a Quattro Stagioni jar. The one-piece lids are indeed very handy, even if they are a bit pricey (but you probably already noticed that).
All of the Bormioli material (website) and instructions (I'm reading them now from the lid packaging) only mention canning with boiling (pasteurization to be precise), and I've only seen them used that way. This restricts the type of foods you can can (pun not intended) to highly-acidic ones: fruit conserves, tomato sauces, pickled vegetables, etc.
Of course, for maximum hygiene you should follow the instructions to the letter, and use a new cap every time. However, I've seen people successfully re-use caps for canning with less spoil-prone foods (e.g. pickled vegetables), but this is anedoctal and I don't think I can really suggest it.
The lid packaging also report a toll-free number “grandmother Amelia info” (not joking), but unfortunately it's late now as I write; I might call next Monday if I have time.
Happy canning!
Best Answer
Metal may be lighter and sturdier, but glass is nicely non-reactive and easily cleaned for re-use. It's also easier to see what it looks like inside without breaking the seal, to see if there is something that doesn't look right. It's a lot easier to do safely in home environments.
I'm not sure if the difference in the thermal properties of the metal ring vs the glass lip are important in water bath canning (e.g., the ring expanding more than the glass).