The Blues weren't as impressive as it sounds. They joined the league in the 1967 expansion, and the NHL set up the 12-team league to guarantee early success for the expansion teams. The Original Six were all put in one conference, and the six expansion teams were all in the other one. That way, there would always be an expansion team in the Stanley Cup finals.
The Blues had a number of great players but they were long past their primes. They made it to the finals in their first three seasons from 1968 to 1970 but were swept every year by dominant Montreal and Boston teams.