I’m not sure if this ever worked, but at least in latest LMTX it doesn’t:

"""
\setupcolors[state=start,cmyk=yes,rgb=no,intent=overprint]

\starttext

\framed[
   background=color,
   backgroundcolor=yellow,
]{\ss\bfd
   Black should always overprint.
}

\stoptext
"""

The black text and frame border don’t overprint the yellow background 
but knockout. The only way to check this (that I know of) is Acrobat 
Pro’s separation simulation; the screenshot shows a snippet of the 
yellow separation.

Actually, setting “overprint” for all colors doesn’t make sense, it 
should be a setting for single color (but default for black):

% desired interface:
\definecolor[overBlack][k=1,intent=overprint]

Hraban