I think that most casual users would expect the title to at least be in bold.
IMO there are way more cases where the user wants the title in bold rather than looking similar to the block content, so I’m in favor of putting it in bold by default.
Note that with the HTML changes proposed in the PR, a block is introduced to contain the title (makes it trivial to apply whatever custom style on it from a style extension).
See for example: Loading... and the discussion on Refactor note, tip, info, success, warning, error macros · Issue #97 · xwikisas/xwiki-pro-macros · GitHub
EDIT: I’ll test different markdowns and make sure that it’s possible to override the new default style for titles easily.