Support for `deblob` USE in kernel-2.eclass is currently broken. Fix: https://github.com/gentoo/gentoo/pull/2953 You'd have known that you're doing deblob thing wrong if you just had asked the upstream for the correct place to get links to immutable versions of the deblob script. After you apply fix to the eclass, add back support for deblob to the kernels. It just works.
Eclass changes go to the gentoo-dev mailing list.
Upstream will release new versions of the script without changing the name of it. They dont have: deblob-4.9-1 deblob-4.9-2 etc They have deblob-4.9 and will release new versions of it with the same name. So, your suggestion would not work.