Toshiba Sat. P25 screen goes black

Do you happen to have a spare monitor, or a cable to connect to your TV? (many modern TV's support this)

You need to test whether your spare monitor or TV "goes black" in the same way your laptop display does. If your spare monitor or TV show your laptop booting okay, then there is a problem with the screen unit of your laptop. (potentially the inverter, or backlight, or in rare instances a hinge switch) But, if the same thing happens, your spare monitor or TV "go dark" too, then it is likely a fault with the video controller on the motherboard.

the inverter and screen are ok , I put them on my second P25 and they worked fine. Can I take just the MB in to be tested?

That's entirely up to you, my friend. :-) But, may I suggest that you check the sensor that detects if the screen is closed?

Something else you should take into consideration, if you take only the motherboard in, you are not allowing them to see for themselves what the fault is, and that is very important when it comes to diagnostics.

Also, if they are able to perform some tests on the motherboard and find nothing wrong with it you may still have to pay them.