Well, many thanks for you answer, Klas!
However, it doesn't really solve the core of my issue. I am aware that it's possible to control parent window with javascript, this part is clear to me.
My problem was the actual workflow to close modal box AFTER the SEBLOD form in iframe is submitted -> where to redirect and where to write the script that will close modal box in parent window.
In fact, I need to achieve the same functionality we can see in SEBLOD backend -> field edition. The field form appears in modal box iframe and after Save & Close the form is submitted and after the modal window closed.
Again, thaks for sudgestions.
Michal