Packages in NonGNU Elpa can only depend on packages in core Emacs, GNU Elpa, and NonGNU Elpa. As such, currently, no package that depends on org-special-block-extras can be included in NonGNU Elpa (nor GNU Elpa).
Adding this package to either NonGNU Elpa or GNU Elpa would alleviate that. Additionally, both GNU Elpa and NonGNU Elpa are enabled by default in recent Emacsen, so adding this package to either would also reduce the friction for new-users.
For situational awareness: Contributions to GNU Elpa require copyright assignment paperwork to be completed. Contributions to NonGNU Elpa don't require copyright assignment.