Most of "Barry Lyndon" was shot with a Tiffen Low-Con #3:
When you get a bright area in the frame, you will notice a bit of halation, though not as prominent as from a Fog filter or a ProMist filter.
I think the Schneider Low-Cons may be a bit more like Tiffen's UltraCons, which lower contrast, lift blacks and shadows, but with less halation.
Low-Cons were popular in the 1970's, along with Fog filters. The past sequences in "Godfather Part II" were shot with Low-Cons. "Bound for Glory" used a mix of Low-Cons and Fogs. Harrison's Double-Fogs were a combination of Low-Cons and Fogs. A lot of "E.T." was shot with a very light Double-Fog.
Tiffen's UltraCons and things like Schneider's DigiCons came along after the 1970's.
Peter Hyams also used Low-Cons a lot, along with smoke, in movies like "2010".
The older filter technology of Low-Cons means that they soften focus slightly and the halation adds a bit of grain-like texture -- some people like this when combined with digital cameras, just as some people like Black ProMists for a similar effect.
In terms of black level, that's something you can control with digital color-correction. Truth is that if you just want a more pastel and lowered-contrast look, that's fairly easy if working with a log or raw original, so I would only use Low-Cons if you want that bit of retro optical halation and softening. These days, something like a #1/4 would be fine but there is no reason not to test.