Not by default. However, there’s an experiemental executive that supported this. If you set the vtkThreadedCompositeDataPipeline as the default executive using vtkAlgorithm::SetDefaultExecutivePrototype and then the executive will execute the algorithm on each block in parallel. I say it’s experimental since I am not aware how well tested/used this executive is.
Better option is to use vtkCompositePolyDataMapper2 instead. It avoids merging all blocks into one.