P.S. Blade length is a complicated issue.
In principle, ideal blade length and shape has almost nothing to do with the boot, and everything to do with your personal anatomy, because, AFAICT, what you mostly want to do, through blade length, shape, and placement, is
1. Place the sweet spot (curvature change point, which is more or less a good balance point) just behind and underneath the ball of your foot, so you can balance easily both slightly behind, and at, the sweet spot. (BTW, a really good skate sharpener can modify sweet spot placement - so if your sharpener is that good, this is a secondary criteria.)
2. Place the back of the blade an appropriate distance behind and underneath the back of your heel. It needs to be short enough that you don't trip on your tails, but long enough to keep skate friction to a minimum, and prevent you from falling backwards on jump landings, and other moves where you might otherwise fall backwards.
3. Place the toe pick an optimal distance (varies by skater) ahead of and underneath the front of your toes, so the amount of toe point your foot needs to reach and jump off of your picks is comfortable and controllable, and just plain feels right. Part of what makes that complicated is that factors associated with controllability vary with both anatomy (e.g., flexibility) and skating skill.
However, because the points of your foot project approximately vertically to points on the ice, if your new boots have a higher heel, you would need a slightly shorter blade, all other things being equal. In addition, most fitters, in practice, mount the front of the blade mounting plate flush with the front of the outsole. While that is quite arbitrary, silly, and isn't always optimal for you, it means that outsole placement on the boot does affect what your blade length should be, so you can't ignore the boot maker's advice completely.
Advanced blades make it easier to reach the toe pick, so it is conceivable, depending on your body, that you might want a slightly longer blade to partially compensate - but I'm not too sure of that. (Mark IV's are very much beginner blades, and are deliberately designed to make it less likely for a skater to accidentally touch the toe picks.)
A really good fitter can try to make a good guess on what blade length will be optimal for you, though there aren't many fitters that good. But if you trust him/her reasonably well, it's a good starting point.
Good luck!