So, recently, I noticed that sometimes the movie gets slightly darker for some seconds and then it gets normal again, this is really annoying, and I'm not sure what it is.

Someone in other forum said I should turn "Dynamic Contrast" off, but the option is not on my TV.

could be connection problem: component/hdmi? i've had an issue like that with one of my consoles... 

What kind of tv is it? When it happens to my projector it is because the lamp is starting to go and needs replacing.

Only other time I had that problem was with Macrovision copy protection not being compatible with my old CRT projector. But that's from the VHS days.

As far as I know you can't play blu-ray over component anymore so bad connection is out. Unless you use composite video?

No, the TV is fairly new, and I'm using HDMI cable.

After making some random "tests", I noticed a few things:

It happens in some specific scenes, and if I pause, it goes back to normal.

It is very subtle, but can get annoying.

It is not the movie, because the whole screen gets darker.

That's really weird. I'm not an expert, so I can't advice anything other than reset both the TV and Blu-ray player/PS3 to factory setting. If it persists try other cables/TVs to see where the problem is and call their customer service.

Sounds weird. Did you try to play the movie with another BR Player ? Just to rule out its the player. If you have an LED TV maybe it switches of the wrong LEDs in some scenes ? It cant be the movie or cable both digital, BR player seems unlikely what TV do you have ? Does it happen on other TVs ?

I don't have any other TVs to try it on. The TV is LCD.

I must assume the TV is somehow trying to adjust the light  in that particular scene, but I'm not sure how to fix it.

The TV is LCD yes but what does it use to illuminate the screen ? Led lamps or the fluorescent old ones.

Rather than an hardware problem it seems more likely a matter of software, like some kind of filter from the TV or form the Blu-ray player.

Check the manual, or optionally go check the TV model, sometimes you can find TVs optimal settings posted on the internet.

If its the software play around with the settings there will be a solution. But maybe you have one of those LED tvs that use mirrors to reflect the light evenly on the LCD screen to be slim and achieve similar quality as the thicker LED LCD tvs. Maybe those mirrors screw somehow up while trying to get the perfect contrast. This can result in certain scenes to illuminate the picture wrong. Would be a Hardware problem then. But this should also happen everywhere in certain scenes, be it TV, Games, Movies.

If its just because of wrong settings you will find a solution.