Diffusion filters, I absolutely love them and use all different kinds. I'll be going over the White Mist & Black Mist from Hoya. These live on my film cameras but you could also use them in your digital SLR cameras, mirrorless or any cameras where you want some diffusion to add a little bit of a character to your images.

I purposely back lit myself and wore a white shirt so you can see the effect of the filters we're going to be comparing. Hoya only makes one version of these diffusion filters. They don't come in different strengths. Above you can see the framing without any filters.

This is the black-mist filter. You can see the blooming a lot more around the edges and any anywhere you see highlights.

This is the same exact settings/exposure and how the white mist looks. As you can see, the Hoya White mist is a lot stronger than the Black-mist.
There's a few things to consider when using any type of blooming or mist filter: 

1. The type of camera you're using
2. The lens / focal length

The camera sensor size will directly affect the amount of halation that you see in your images. The smaller the sensor the more more blooming you'll see and the bigger the sensor the stronger the strength the diffusion filter you'll need to get the same amount of blooming. Focal length also directly affects the amount of diffusion too. If you're using a telephoto lens like a 200mm, that's going to need a stronger amount of diffusion then a wider angle lens like a 24 mm for the same amount of diffusion. 

For me, the white mist filter permanently lives on my Leica m3 lenses when I'm shooting film. However the effect there might be a little bit difficult to see because I shoot really stylized with a lot of grain and a lot of contrast. I also use the white mist filter on my medium format camera when I shoot film. Here you can see how the effect of the white mist filter isn't as strong on 6x6 film vs sample (above) shot on the crop senor of the Canon m6mk2.

Personally I prefer the White Mist filter because all Black Mist filters affect the temperature a little bit in your images. Generally what Black Mist filters tend to do is warm up your image a bit. Below you can see I ran some tests in a control environment and you can see how exposure and filters affect the image.

We can see a light fixture directly in the frame and how the filters affect the blooming. We can also see how the Black Mist filter adds warmth to the blooming in your light source and your overall image.

And here's just another quick test to see the different type of blooming I did by just holding my hand out directly in sunlight. What diffusion filter also do is make your highlights glimmer. You can see the different amount of strength between a White Mist and the Black Mist.

I absolutely love these filters. They live on my film cameras and they're a great way to add character to your images if you don't want something really clinical or clean looking. These Hoya Mist filters will give your lenses some style and character.

Filters used:
Hoya 67mm White Mist Filter
https://bhpho.to/2NCsimz
Hoya 67mm Black Mist Filter
https://bhpho.to/3pw25U6