Light blocking your model is one of those tasks that can seem daunting if its your first time but in reality is quite easy to do.
How you do it depends on the type of model you have - is to be internally lit like windows in the Starship Enterprise or cabin lights in the hull of a boat where we want the light to reflect internally for better effect, or is it an aircraft or vehicle with some interior lights?
Some models have thick enough plastic that light blocking isn't required. Games Workshop models for example are very thick grey plastic and light doesn't tend to bleed thorugh. You may have some parts that don't fit quite as accurately as they should that need filling to prevent light leaks, but otherwise you won't see the light shinning through the plastic itself.
It's most prevalent with very thin plastic model shells where the intense LED light shows though from inside rather than just through the windows.
So, how do we light block these models?
Example: Starship Enterprise.
Almost totally internally lit with clear plastic parts for windows we want to be lit.
With this type of model we generally use a lot of LED strip run round the interior of the saucer and hull to light up the windows, relying on bounced light to create the right type of diffused lighting. We can't put the strips infront of the windows or you would a) see the strips and b) the light would be dazzling.
To light block these models is very easy becasue they are never seen inside.
First mask off the edges of the model so you don't get overspray onto the outer surfaces then spray the interior of the model with matt black spray paint. You want 2-3 even mist coats until there's no plastic visible. We do not want drips of paint or thick areas. Just lightly spray even coats until it's covered.
To allow light to bounce around inside spray over the black with a couple of mist coats of matt white spray paint. Enough to cover the black but doesn't need to be a thick coat.
And that's it. The light won't penetrate the black paint and the white top coat reflects the light around the saucer and hull so we have a nice lighting effect.
Photo below shows the intial light test. No window parts are installed so the lights are quite bright, but as you can see, no light is shining through the plastic itself.
Example 2: A vehicle or building with internal lights
In this example we don't want the light to bounce around inside so much. We just want to stop the light bleeding through the plastic shell of the model so we can see the interior features.
Here we would just need to apply the black mist coats, preferrably to the interior of the model and this will absorb and block any light so no light can bleed through to the exterior.
You would then apply any interior paint details over the top and the lights would highlight the interior details without bleeding through to spoil the model.