You are fine with 18". In fact on the CX-9 you can go as far down as 17". This is what my dealer here recommends for winter tires.
Sometime online they will say that tires won't fit because the difference in load rating or in speed rating is too large from the original tires. I bet the blizzak as a lower speed rating than the other tires proposed online and that's why they show the blizzak as not available.
That being said if you plan on using steel rims, I would recommend going with a tire with a narrower profile (either in 18" or in 17" inch). I did just like you and bought 255/60/R18 Nokian, but it never occurred to me that the 18" steel wheels are almost all 7" wide, and a 255 wide tire is better suited to a 8" tire. For example 255/60R18 Blizzaks DM-V2 states that they will fit on Rims 7.0 to 9.0 inch wide. So they technically are safe to use on 7 inch, but that is the bare minimum wheel width that they will fit on. I was never warned about this and finding out steel rims width is not always easy.
On my Nokian, the tires sidewalls are really round and I am having a hard time getting them balanced properly, it could be bad wheels but two of them were already replaced and I got the tire rebalanced multiple times. I am now wondering if that's because of the narrow rims. I was planning to either sell them and change them for narrower 17" or buying wider 18" wheels, but I didn't have the funds for it this fall.