Similar to Jose Quintana, Paul Blackburn should not be the top starter they bring in, or even the second-best starter they acquire, but he can be a really good back-end of the rotation option for St. Louis.
Blackburn is under club control through 2025, which would give St. Louis a cheap starter for the back of their rotation for multiple years. If they are going to be spending big on top-end starters, getting cost-controlled guys for the back end makes a ton of sense.
Blackburn isn't flashy, striking out 9 batters per 9 innings to the tune of a 3.81 ERA. But that kind of production, especially from the back end of the rotation, would be a huge upgrade to this staff. I don't think Oakland would be in a hurry to move Blackburn, but the cost to acquire shouldn't be crazy either.
The five options I laid out each have their pros and cons and fill different needs in the Cardinals rotation. The main similarity they all share is they are likely attainable without breaking up the best members of the Cardinals' young core. Some of them would still be costly, but overall, this group provides a path where the Cardinals can upgrade their rotation through free agency and trade this offseason without losing the strength of this club.