In contrast to class diagrams, it seems one can model multiple facets of a system in block diagrams such as software, hardware, entities, etc.
Is it possible that a single block can represent both software and hardware? Or would they always be separated into two separate blocks? E.g. When modeling some machinery, let's say the machinery has a physical button, and the machinery's software also has a "Button" software class. Would they be modeled as separate blocks or as an individual block?
The same question could be asked of a database entity that is mapped to an object oriented language as a class, and said entity also represents a real life physical actor (E.g. "User" software class & Real life physical user).
In the case that a single block can represent multiple facets, is there some form of notation to indicate "this block represents both hardware and software" - or would this just be implied based on the block having multiple and differing annotated relationships with other blocks?
A systems engineer would generally want to separate physical components and software components. The "digital twin" in software does not always represent its physical object accurately. For example, bad sensing may cause a digital twin be an inaccurate representation of the component in reality. Imagine an autonomous vehicle's position in traffic, or a "stalling" 737 MAX.
In an information system, the digital twin for a person is different from the actual person it represents. Imagine what would happen if your doctor only treated you according to inaccurate health records. You, the actual person, should be distinguishable from your health records. Moreover, the design of software should be distinguishable from the design of a database schema so the two can vary independently.
To answer your question, there's nothing to stop you from modeling everything as one block, but, if you conflate things in a model, you can't reason about them separately. Why would you want to model a physical thing and its digital twin, or a Java class and a database table as the same SysML block?