A recent change in revision history states this was solved back in 2017, however at this time of this post there's still people having recent issues with playing online with it not working- https://steamcommunity.com/profiles/76561197970542629/recommended/235600/. Myself and another friend have also encountered these same issues with going in circles with Ubisoft support trying their suggestions to no avail. Even trying the fixes that were listed here before gave no results. Seeing as this seems to be an that is showing for some and not others I believe the section should be revised, not removed.
Topic on Talk:Tom Clancy's Splinter Cell: Blacklist
Blacklist online issues section removed prematurely?
> A recent change in revision history states this was solved back in 2017, however at this time of this post there's still people having recent issues with playing online with it not working
> Even trying the fixes that were listed here before gave no results.
The above basically describes two very different issues. The section that was removed pertained to a specific older version of Uplay, which was eventually solved the same year, and the page had it listed as a "fixed" issue -- not an unresolved one -- because the listed workarounds solved it. The section had served its use and was not removed prematurely.
That said, we can of course add an unresolved issue section with this "particular" issue beneath it, but based on the lack of information pertaining how to fix it or that it is even widespread it would basically look something like this:
Cannot connect to multiplayer games
And at that point, I can think that warranting its inclusion to be... a bit hard to argue for. The above results is from the first page of search results when searching for "multiplayer" on the Steam discussion boards, mind. I excluded results from 2019 or earlier.
@OP Can you try to use a VPN along with your friend (a one w/o blocked ports at best)? Eventually mobile connection to exclude problem with provider? Probably you also tried to disable firewall, enabling UPNP on router. Generally keeping NAT open/necessary ports open/forwarded only if your provider doesn't block it (mobile networks usually do it unfortunately).
i can confirm coop is broken for Comcast ISP and no amount of port forwarding will fix it.