Of course it's just a coincidence, the only time Bernier and Blackwood played well was the first ~15 games of the season, which Jack almost entirely missed.
Jack Hughes was injured in the 1st period on October 19th. From that game until his return (November 30th), our cumulative all situations SV% was .9111%, which is below average (19th in the league) but not horrendous. We were 20th in GF/60 and 19th in GA/60 during that time which is bubble team territory.
Since December 1st our cumulative all situations SV% has been .8719%, worst in the league. We're 17th best in GF/60 (slight improvement) but 30th in GA/60.
This is despite our shots against/60 not really changing since then, we allowed 30.9 SA/60 with Jack out compared to 30.7 SA/60 since he came back.
I'll also point out that the rest of the team remained relatively healthy while Jack was injured - Hamilton missed three games, Sharangovich and Smith were healthy scratched a few times, but pretty much all of our regulars had perfect attendance.
Since Jack has been back, Siegenthaler and Mercer are the only ones with perfect attendance. Hamilton has missed 16, Hischier has missed 6, Graves has missed 5, Bratt has missed 3. And obviously the goalie injuries too.