TextBlock should not be sealed
Sealed classes fly in the face of OOD, so there should be an incontrovertible reason for making a class sealed. I don't accept the rational that sealing the class makes it perform better. If you don't want the performance hit of virtual methods, then don't make them virtual. There's no justification for sealing the entire class.
Now that JS officially dropped out of support, I think that MS should consider removing this very annoying limitation.
Don Airey commented
There's a great discussion on the subject here: https://stackoverflow.com/questions/2134/do-sealed-classes-really-offer-performance-benefits. The community has looked at the subject, found no benefit to sealed classes and generally agrees that it corrupts the intent of OOD.