It should be possible to configure the info.magnolia.editor.block.stock.RichTextBlock whether it should provied browser-based spell checking or not. By default they should provide spell checking.
The blocks in the stories app should use this default and support browser-based spell checking.
Since the RichTextBlock uses the ckeditor- maybe the following link is helpful
We should also consider if there are other options that we want to make available on the RichTextBlock.