INT 10 - VIDEO - TOGGLE INTENSITY/BLINKING BIT (Jr, PS, TANDY 1000, EGA, VGA)
        AX = 1003h
        BL = new state
            00h background intensity enabled
            01h blink enabled
        BH = 00h to avoid problems on some adapters
Notes:  although there is no function to get the current status, bit 5 of
          0040h:0065h indicates the state
        when configured for a monochrome display, the Boca Research Multi-EGA
          with ROM v M1.1 Type D has its screen disrupted if BH is not clear