For the near future, the library’s stance on marks is that you probably only want to use them on inline nodes, and that’s what most of the mark-related library code is written for, but putting them on block nodes is allowed, and though you won’t be able to use utilities like addMark and removeMark directly to manage such marks, the lower-level code should handle them properly. It may not always do so yet—this isn’t well-tested territory, so file bugs when you run into problems.
I do reserve the option to change my mind on this in the future, but for now it seems like allowing block marks isn’t really in the way, and can be useful to some people.