Yes, Add blending mode works best against dark backgrounds. In your case you should start by color correcting your footage. You jsut can see that it's overexposed and the Gamma completely off. Recovering some contrast will already go a great ways toward improving perception of the light sabres. Beyond that play with otehr blending modes and additional adjustments on the light beams themselves...
It is a simple matter of math. Looking at the footage you haven't got any range to work with. The average luminance brightness of the background is above 95%. That only leaves you 4 or 5% of the upper range of the colors to use for your light sabers. That's not enough. Take a look at the histograms and look at the RGB values from this screenshot.
Compare the histogram and RGB values from this frame:
The sample is taken from just to the right and slightly above the hilt of the light saber. In this image there's lots of room to work, in your footage there isn't. It's all used up. So the answer to your question is already answered, kind of, by you. You must darken your footage. You don't use opacity, you use one of the color correction tools like Exposure. If the footage you've showed us is the way it came out of the camera then you really need to reshoot.