How to create entity within blockForm?
The easiest idea I could come up with would be to define the paragraph type through the UI. Then you'd have 2 options: 


whether make a custom form with similar fields and create a new paragraph entity on form submission.
define a content type which has reference field to the paragraph type. Then you could simply render the node form which would handle all the logic of displaying the fields and saving of the paragraph it self.


I would prefer option number 2 since paragraphs are not meant to be stand-alone entities. They are meant to be parts of content. IMHO I don't think one paragraphs entity should be reused on multiple nodes.