It seems like people who favor Morris over Blyleven say Morris was better in the clutch or better in big games. So I try to look at those issues here.
The table below shows their stats in 3 situations: runners on base (ROB), runners in scoring position (RISP), and close and late (CL). Data from Retrosheet.
I did not try to adjust these numbers for the league average. Blyleven might get a slight edge since the early 70s were not a big hitting era. But much of their careers did overlap. The only place where either pitcher has a big edge is Morris's edge in AVG in CL situations. But that .021 does not add up to alot. Blyleven had 2,129 ABs faced in those cases. That amounts to about 44 hits or 2 per season. That seems pretty small.
The next table shows their post season stats. League Championship Series and World Series are combined.
Morris has just about twice the IP. So if you doubled Blyleven's stats, you can see that there is not much difference between the two. Blyleven would have 86 hits, just about what Morris has. Same for HRs. But he would have more strikeouts and fewer walks.
I also looked at how they did in September pennant races. If a team finished 10 or more games ahead or behind, it was not considered to be a pennant race. If a team finished less than 10 games ahead or behind and if they were 5 or fewer games ahead or behind at the end of play of Aug. 31, it was considered a pennant race. 1991 for the Twins was not considered a pennant race (Morris was on that team). They began Sept. 7 games ahead (GA). On Sept. 15 they were 7.5 GA and they finished the season 8 GA. 1981 was not included since it was a strike year with a split season. Many teams were within a few games in Sept. This is highly unusual and winning the 2nd half only gave you a chance to play for the divisional title.
So the years I have for Morris as Sept. pennant races are 83, 87, 88, 92, 93. For Blyleven they were 77-80, 87, 89. Each pitcher had a total of 231.66 IP (Oct. data was included). Some of this data might inlcude games pitched after the divisional title was decided. But I did not feel like spending the time to figure that out. The table below shows how each pitcher did in these cases.
Again, it does not look like there is much difference between the two. So given Blyleven's far superior career stats (and peak value as measured by stats like WAR), he still deserves to make the Hall of Fame ahead of Morris. Whatever edge in the clutch or big games Morris might have, it is definitely not enough to put him ahead of Blyleven.