Nope. Theoretically both can be simultaneous. Grulla is a completely different gene than roan, and yes, they are both black based. Thus she carries both genes (roan and dun), similar to how a dunalino carries both cream and dun.
Yup. Grulla roan is entirely possible. And that is what this horse is. Remember, a grulla is a black horse with a dun gene. A blue roan is a black horse with a roan gene. This horse is a black horse with both a dun gene and a roan gene. This is why it looks like a blue roan.
I had to laugh a bit when they said "she will definatey produce color". Spelling mistake aside, it's a silly claim! She's a very pretty colour, though. And yes, it is entirely possible to be grulla roan. It's just a black horse with both a roan & dun gene.