I'm thinking cellulose - most of what I see is either that color or a dark gray color.
You didn't mention, but I'm sure you got, the insulation around the vent/flue. Looks like a nice insulation baffle around that vent/flue ... sure did keep the insulation in around it pretty darn good.
I don't see it separated either, looks like a fire stop support with a support ring around the vent/flue.
What the opening does do, though, is allow you to see that there are no supports holding that vent/flue (other than the fire stop with support collar). As I under it, each section needs to be supported???