What I've found is that this well-used blade doesn't cut less effectively, when used appropriately, but rather it is less dangerous. I seem to get less skin irritation, and I can use techniques such as buffing strokes that might be ill advised with a new blade.
Take this morning, for example. I used my open-comb Merkur 15C razor for most of the shave. I lathered well with my own, proprietary soap, and made the first pass with slow oblique buffing and an anti-raking stroke pattern. I did not rinse the razor for the entire pass, which left more than ample lather on my face and under the razor to eliminate the need for more brush-to-face lathering. I simply re-wet my face with water and used my hand to spread the used lather uniformly again.
Merkur 15c open comb razor
Second pass was largely across grain above my jaw line, and against grain below my jawline. Again this was with slow oblique buffing strokes and anti-raking stroke pattern. Also once again I didn't rinse my razor, merely re-wet my face, and again spread the residual lather from face and razor uniformly over my face.
Then I took a third pass against grain with the 15C.
Not quite satisfied, I transferred the blade into my finishing razor, the Weishi 9306-F, and, after again wetting and spreading the now-thinning residual lather (having sufficient water is the key), did a fourth pass, once again against grain.
The result was an extremely close, surprisingly comfortable shave. A post-shave alum rub revealed surprisingly little latent irritation. I rinsed the alum off with a witch hazel rub, dried my face, and applied the last of my Nivea balm for sensitive skin. Then some unscented moisturizer over my entire face (except for my nose).
Had I done this process with any new blade, you can bet that I'd have likely had weepers and skin irritation galore. I think that the old blade plays a key role -- but not just because it's old.
I must reiterate that I groom the blade after each shave by first rinsing and drying it, then oiling my palm and stropping the blade on that oiled area. Also, the open-comb Merkur razor, though having infinite shaving capacity due to its open-comb baseplate, is obviously a mild shaver. Paired with a fresh blade it is far from my favorite razor as a final-pass instrument, but I find it excellent for with-grain and across-grain passes. Yet, with a well-used, well-seasoned blade like today's Bluebird, it was comfortable but not dangerous even on my against-grain pass.
And for the ultimate in safe finishing razors, the Weishi 9306-F is unparalleled. Though I don't like this for most early passes in a shave, I might be tempted to give it a try when using a new blade ill suited to my face such as a Feather brand.
Tomorrow I'll be using this same Bluebird blade in my vintage Gillette Tech razor. It's unlikely that I'll take four passes, but I do expect the shave to be face friendly. We'll see....