Line a 9 x 13 inch rectangular dish with plastic wrap and lightly brush on the vegetable oil.
In a large baking tray, sift together the sugar and cornstarch, and cover with plastic wrap. You will need this later.
In a medium saucepan, sprinkle the gelatin on the blueberry purée, and let sit for 5 minutes. This will allow the gelatin to soften.
Add the sugar to the saucepan, and bring to a boil*. Stir the mixture to dissolve the gelatin.
Remove the pan from the heat and cool for about 3 minutes.
Pour corn syrup into the bowl of an electric mixer (i.e. KitchenAid) and turn on to a slow speed. Quickly add the slightly cooled syrup in a thin stream, as the beaters are moving. Once all of the gelatin mixture is added, whip for 15 minutes on a high speed. The mixture will increase in volume, and become light and fluffy.
Spread the mixture evenly in the prepared dish, and leave for about 6 hours to set. You can also chill the mixture in the fridge for about 3 hours.
Once the marshmallow has set, turn out onto the prepared tray (the one with the cornstarch and icing sugar).
Use a bat shaped cookie cutter to carefully cut out the marshmallows. You will need to rinse the cutter under hot water frequently in order to get clean cuts. Be sure to dry the cutter thoroughly after washing.
Dust each marshmallow bats liberally with the cornstarch and icing sugar mixture from the tray, this will prevent the bats from sticking to each other.
When you're ready to decorate your bats, make sure they are laying on a flat surface. You can't move them until the glue dries, so make sure you're happy with your set up before you start.
Dab a tiny amount of the edible adhesive behind each eye, and press down gently on to the marshmallow. If you press too hard, you'll make a hole in your bat. If you use too much glue, it will leave a smear on your bat. Also, be careful because the adhesive (and water) will cause the black on the eyeballs to smear. Work slowly and carefully for best results.
Use the black food writer to draw a mouth, teeth and/or wings on your bats. You could also use royal icing to add dimensional details to your marshmallows.
Allow the bats to dry throughly before eating or storing.
Store in an air-tight container up to one week.