Don't do them separately.. do them together one after the other directly.
You could also try to change the order of the images if the above doesn't work. Although that is not the normal procedure.

When you flash.. do you see the usb and Nokia logo? It is very faint but can be seen better infront of a lamp. If you do not see anything it could be a hardware failure..

You could also try to connect it to your tv.. if this works and the screen on the N900 still is black then it is probably hardware.

if you still have guarantee try to use it or buy a new screen and switch. They are not that expensive and fairly easy to change. Do not try to find the cheapest spare part though.. they are mostly junk.