The technical term System is relative and depends on the viewpoint. From one viewpoint an entity is a system, from another one it is a subsystem or an external system. It is a role that is applied to an entity. You loose this flexibility of changing the viewpoint if you model a system context with SysML and the model elements Block (for the system) and Actor (for the users and external systems). By definition, the model element Actor represents an external entity and could not be a system in another viewpoint. That’s often a problem in system modeling. On the other hand, a system context is a key artifact that could not be skipped. I propose to keep the concept of system actors and to count out the model element Actor in system modeling. Use stereotypes like <<user>> or <<external system>> to manifest the concept and specialize them from the SysML element Block instead of extending the metaclass Actor.
The icon for the actor like a cuboid for external systems or a sticky man for users are defined at the stereotyped element. Therefore a system context diagram with actors based on the model element Block looks the same as with actors based on the model element Actor. In addition, you gain the opportunity to model ports and parts at the actors, e.g. to get a more detailed description of the actor/system-connection.