This was an issue in the upstream project, not in the PKGBUILD and has already been fixed for a long time now.

Can't get this package to install. Only can install it through manually compiling it with the instructions on the gitlab page.

The only error (when installing through AUR) is that it can't find the blueprintcompiler python module

blueprint-compiler requires gobject-introspection to work. So, it should be moved from makedepends to depends array.

And python dependency should be replaced by python-gobject since blueprint-compiler depends on it to work.

depends=('python-gobject' 'gobject-introspection' 'gtk4')
makedepends=('git' 'meson')

There are no tags available, please include the revision as well:

pkgver() {
  cd "$srcdir/${pkgname%-git}"
  printf "r%s.%s" "$(git rev-list --count HEAD)" "$(git rev-parse --short HEAD)"
}

r80.d0cf13b