I watched the video LASER Academy, and I've been wanting to know how to make that digital muzzle flash look.
Those are regular old muzzle flashes colored blue. In Hitfilm you could recolor flashes using Curves, Levels Histogram, Gamma, Color Balance, Channel Mixer, Hue/Saturation/Brightness, Color Wheels, Color Brightness, Gradient Map or a few other effects. In Express some of the listed effects are Add-Ons.
Probably the easiest is Gamma. For a blue tint try pulling down the red and green sliders.
Let's talk Gamma, Levels Histogram and Curves for a moment. Curves is the most powerful color correction tool. Period. And a great thing about Curves is it works exactly the same way in every single program. So, while you won't find many Hitfilm specific Curves tutorials, ANY Curves tutorial for ANY program works exactly the same way in Hitfilm.
Picture a two-axis graph. The bottom axis from left to right represents the "input" channel value from left (0) to right (full) - the values your frame always has. The vertical axis is "output" channel value from bottom (0) to top (full). This is the final value after the filter is applied.
The default curve is a diagonal line, bottom left to top right. Clicking on the line adds control points (and you can move the endpoints). Moving a point UP increases brightness, DOWN decreases brightness.
Watch almost any tutorial and the chances are you'll see Curves used in color correction. You'll often hear something similar to "I'll use Curves to add contrast." Well, that's adding a point about about the 1/4 and 3/4 parts of the line. Drag the lower left point down the upper right point up. This darkens shadows and raises highlights without clipping.
The short version here is to go watch Curves tutorials. Curves can adjust brightness, contrast, color casts, invert color channels, creatively color things, and much more. If you learn Curves you learn Levels Histogram and Gamma.
Levels Histogram is... Curves! Instead of adding your own custom points Levels has a none, full and mid point for each channel. Moving the black/white points is moving the ends. Moving the gray point moves the center. Levels Histogram has one issue.... The Histogram function requires readback from the screen to the Histogram. This makes the filter slow performing and rendering. Use curves. The only reason to use Levels Histogram is if you know how to read a Histogram... It's the only way to get any "Scopes" display in Express without the Add-on.
Gamma is even simpler. Gamma has a fixed black/white point. All Gamma has are three sliders and all they do is move the center point of a Curve!
What about the blocky outline and the hit?
here's what you told me https://www.youtube.com/watch?v=TMy04u_tTFA&