That's right, and that's - for now - on purpose. Single blocks, scripts and script pics don't include customized primitives, but instead reference them by name only. That way importing them into another project will not change the target project's primitives. This is kind of a big deal, so looking at someone else's code doesn't change your project in ways that you can't easily track let alone undo.
There is - as of now - one exception: Snap! does support customized primitives when you create a library.
In general I'm as much excited about folks (like you) who love tweaking their own blocks and changing primitives, as I'm upset about it. I'd much, much, much rather see folks program interesting projects that do interesting things than relentlessly fidget around with variadic inputs and unnecessary metaprogramming. That doesn't mean that I want you to stop, but I'll likely start to push back on reports such as this one if I find the community in this forum to obsess too much over highfalutin' pseudo-advanced metaprogramming stuff.