From the Blues' press release:
The St. Louis Blues and FOX Sports Midwest announced today the club’s 2015-16 regular season television schedule. All 82 Blues games will be televised for the fifth straight season.
FOX Sports Midwest will televise 70 regular season Blues games, in its 20th season as the Blues flagship television partner. The first eight games of the season air on FOX Sports Midwest, including the season opener vs. Edmonton on Thursday, Oct. 8, at Scottrade Center. Blues games on FOX Sports Midwest are preceded and followed by Blues Live, the network’s in-depth pregame and postgame show.
Viewership for Blues telecasts on FOX Sports Midwest has grown each of the last five regular seasons to an all-time-high 4.9 household rating in 2014-15, according to Nielsen Media Research. Blues’ local market TV ratings ranked second among U.S.-based NHL teams, and prime time games on FOX Sports Midwest ranked No. 1 vs. the prime time averages of all other channels in St. Louis among Adults 25-54 during the 2014-15 season.
TL;DR - FSMW will broadcast all games not shown on NBC or NBC Sports Network.
The most important part of that press release is the last paragraph. A 4.9 household rating for hockey is pretty not bad. Being the second highest local TV ratings among US-based teams is also not bad (keep in mind it's based on household rating, not necessarily total viewers). The Blues' continued on-ice success has spurred a ratings bonanza for the team and for the network, which is also something that needs to be taken into consideration as a "what if" factor if the team isn't successful this postseason.