Mix the blackberries with the sugar and let sit for 30 minutes.
Meanwhile, oil the ramekins with the grapeseed, or any neutral-tasting oil. You can skip this if you are planning to serve the panna cotta as a parfait, and not pop it out of the container.
Heat the blackberries-sugar mixture with the cream and bring it to a steam, about 170°F. Don't let it simmer. Turn off the heat and let this steep 30 minutes.
When the 30 minutes is about done, mix the gelatin with the cold water in a container you can pour from, like a quart measuring cup or a bowl with a pouring spout.
Heat the cream mixture again, just back to the steaming point. Don't simmer it.
Set up a fine-meshed strainer over the container that has the gelatin in it. Pour the hot cream through the strainer into the container so you leave the blackberries behind.
Now, pick up the strainer and shake it gently over the cream to release some extra blackberry goodness. This is important, as this last bit of juice and cream in the blackberries is concentrated and can give you a lot more flavor and color in your panna cotta.
Stir the panna cotta mixture a few times to incorporate the gelatin, then pour it into your ramekins.
Put in the fridge for a few hours to set. Panna cotta can be made up to a day ahead, just cover the ramekins once they've set.