I think this should work.
Use this in a script call.
$game_map.screen.start_fadeout(duration)
$game_map.screen.start_fadein(duration)
Example:
$game_map.screen.start_fadeout(200)
If you want to change it for every time you use the command you would have to change it here.
#==============================================================================
# ? Game_Interpreter
#------------------------------------------------------------------------------
# ????????????????????????????? Game_Map ????
# Game_Troop ????Game_Event ??????????????
#==============================================================================
class Game_Interpreter
#--------------------------------------------------------------------------
# ? ??????????
#--------------------------------------------------------------------------
def command_221
Fiber.yield while $game_message.visible
screen.start_fadeout(30)
wait(30)
end
#--------------------------------------------------------------------------
# ? ?????????
#--------------------------------------------------------------------------
def command_222
Fiber.yield while $game_message.visible
screen.start_fadein(30)
wait(30)
end
end
Change where it says.
screen.start_fadein(30)
wait(30)
The 30 to the new fadein or fadeout default duration.