There is no default keystroke for Screen Curtain.  If yours is set to down arrow, it might be that you tried to assign a keystroke and accidentally assigned down arrow?

To set (or remove) a keystroke for Screen Curtain (and I'll list the steps without using down arrow for you, just in case):

1) Press NVDA+n to open the NVDA menu

2) Press P for Preferences

3) Press N for Input Gestures.  The Input Gestures dialog appears.  From here you can either navigate the tree view of commands or filter to a specific command.  To use filter, press shift+tab then type the name of the command.  Note that there is now an option to reset ALL custom gestures back to their default.  To access that feature, from here, press alt+d for "Reset to factory defaults", then alt+y to answer "yes".  However, I'll walk you through browsing to the Screen Curtain command next:

4) Press V to jump to the "Vision" options.

5) Press right arrow to open the vision options.

6) Press T (or down arrow) to "Toggle the state of the screen curtain").

7) If you have assigned down arrow, press D to move down to the down arrow assignment

8) Press ALT+R to remove the assignment.

9) Press ALT+A to add a new keystroke

10) Press ONLY the keystroke to add.

11) Press ENTER to assign the keystroke for only the current keyboard layout, or down arrow then enter to select ALL layouts.

12) Press ENTER to close the dialog.

13) Press NVDA+control+c to save the NVDA configuration.

Kind regards

Quentin.