I got some help fixing my workaround script on the official forums and Monika popped up with some deep knowledge about the precise conditions under which Illustrator will refrain from making a blend spline (https://community.adobe.com/t5/illustrator-discussions/why-is-my-script-undoing-half-its-work-before-it-s-done/m-p/15442971#M448771):

A) If there are at least three paths selected, and at least one of them is an open, unstroked, unfilled path, and at least two of them are not, then the open unpainted path is used as an initial spine and the other objects are moved onto it.

B) If the bounding box of the anchor points of each object encloses the bounding box of the anchor points of all the objects above it in the stacking order, then no spine path is created. (This is the ‘nested objects’ case.)

C) all centers are enclosed in the intersection that is built by of all objects

D) If all of the objects are open paths.

A is *actually kind of useful*, or would be if more than about fifteen people who have been using Illustrator for half their lives knew about it.

B explains some of the times my workaround script fails to work. I can see how it'd be useful sometimes but I'd turn it off if I could.

C, I'm not quite sure what Monika meant by that.

D is the behavior I absolutely loathe because the vast majority of my blends are me saying "see these two open paths? make a bunch more like them". I really can't even imagine a way not making the blend is useful, except maybe "it is 1989 and placing blended objects along a curved spline is an expensive enough operation that we'd love to avoid it whenever possible".

Let's maybe get switches for *all* of these behaviors somewhere in the Blend tool's settings, so that people can learn about them without shipping a copy of Monika's brain along with every copy of Illustrator? And for the "always knockout the blend after creation" behavior too.

Or at least *document* some of this, was this ever in the manual? It's sure not at https://helpx.adobe.com/illustrator/using/blending-objects.html?x-product=Helpx%2F1.0.0&x-product-location=Search%3AForums%3Alink%2F3.7.2-dev.2

THIS IS STILL HAPPENING AND I STILL HATE IT SO MUCH

Also why the @#$% does Illustrator insist on checking "knockout group" on my blends, I *never* want this, I *always* have to go in and turn that off every time I make a blend of a couple of objects with complicated appearances. Gimme a switch for that behavior.

Somewhere in the time between my last comment here and now I wrote a script to close the first path, make the blend, and open the first path, and it worked for a while, and then it stopped working, and now it attempts to close *all* the paths and then restore their original state and I *do* get a spline now but it's not reliably re-opening my paths, and I am just *so @#%ing sick* of working around these annoying behaviors.

Please fix this.

I made a script to work around this. After a while it stopped working. I got it mostly working but it's still got some glitches. I'm so **** tired of this behavior.

Also happy slightly-belated seventh birthday to this feature request.

It's about two years later and I still really want this switch, I still swear every time I make a blend and then have to either eyeball making my own spine, or undo, close one path, make the blend, and re-edit that path.