Use \setupblank[fixed,line].
Aditya's solution sees to force content at the bottom of the page to stay on the current page whereas Wolfgang's forces it to the next page. Both have benefits to me, so thanks for both solutions.
You should get the same output when you change the \setupblank setting to \setupblank[fixed,big].
The advantage of Aditya’s solution is that it will also work when you write \blank[<value>]
because the \fixedblank is applied to each command while the \setupblank solution works
only when you use \blank (without argument), for different values you have to add “fixed”
to the list of arguments (i.e. \blank[fixed,…]).
The disadvantage of Aditya’s solution is that i changes settings from the core which is never
a good idea because commands can change, with \setupblank this doesn’t matter.
Wolfgang