Since having the visual feedback of the LED blinkers in particularly important for heated seats, I decided to just test it out and answer my own question.
Answer: The stickers block too much of the light to clearly see the blinkers except when it’s dark inside the car.
The white portions of the stickers let some light through, but the black portions pretty much block the light.
Here’s a gif showing it in the relative dark of my garage — pretty easily visible:
View attachment 928950
But here’s another gif with the car pulled onto the driveway at midday on a overcast day (so not even any direct sunlight) — very hard to see:
View attachment 928951
Based on that, I opted not to put the stickers
on the buttons, and instead put them
next to the buttons:
View attachment 928955