A diode would be better than a resistor as that would allow d10 to fully pull the line
going to the base of the transistor low (which eliminates the current going into it)
but would not drive the line high which is where the issue is.
An issue with using a resistor will be that while the resistor will protect the pin from
the transistor drawing too much current when D10 is high,
it creates a voltage divider with the existing pullup resistor so the signal going
to the transistor will be some voltage above ground that may not turn off the backlight
as it will allow some amount of current to flow into the base of the transistor.
The actual current going into the base will depend on the value of the resistor
you select, and value of resistor used on the shield as a pullup and the transistor
used on the shield.
Since most shields don't provide an accurate schematic with exact parts used,
it's not possible to precisely calculate a resistor value or tell if it would completely turn
off the backlight (i.e. it might still be very dim, when you try to turn it off)
For sure 125ohm or larger would protect the pin from excess current
but depending on the other components, it might not fully turn off the backlight.
If you wanted to experiment a bit, you could use a pot rather than a fixed resistor.
Then when D10 is low, turn down the pot until you see the backlight go off.
(there is no overcurrent danger in doing this)
You then need to see if that value is large enough to protect d10 from over current.
To do that you need to measure the current going through the wire when D10 is raised to high.
Since the resistor value of the pot might be low enough to allow excessive current,
you should carefully do this test.
Just set it all up, but don't fully connect the meter.
Then connect the meter. The backlight should light up and you can
measure the current coming from D10. If it is over 40ma, the resistor value
should be larger.
While there is probably a resistor value that works acceptable, I'd use
a diode instead as it won't waste additional power when the backlight is on.
Is there a specific diode I should use (ie could you link me to a good one)? And I can just connect it between d10 on the shield and the arduino? Does the orientation matter?
I truly apologize for the simplistic questions.