Hello,

I start emacs -Q (under openbox in X on Debian if it matters), and I get
an Emacs where the cursor doesn't blink.  Thankfully only on master but
not with an emacs-26 build.

The problem is simply that `blink-cursor-mode' is not turned on when
starting, but I can successfully enable it.

The reason is that at the moment the :init-value expression of
`blink-cursor-mode' is evaluated, `window-system' is bound to nil,
i.e. when installing the following: