This will send an event called "focus_on" to Blink(1) whenever a FocusTime session starts. You will need to define a pattern for the "focus_on" rule on your Blink(1), and you most likely will want to create a second recipe to handle the corresponding "focus_off" event when a FocusTime session is finished.
Discover more time saving integrations for blink(1) and RescueTime