Hi,
It seems there is a bug with the command \blank in the latest standalone
context.
If you compile with ConTeXt (texlive 2015), the result is as expected.
Thank you,
Fabrice

\setuppagenumbering[location=]

\setupcolors
  [state=start]

\definecolor[MyColorB][c=0.00, m=0.62, y=1.00, k=0.00]
\definecolor[MyColorA][.75(MyColorB,white)]
\definecolor[MyColorC][c=0.3, m=0.00, y=0.00, k=0.05]

\defineframedtext
  [MyDefinition]
  [before={\blank},
   after={\blank},
   frame=off,
   background=MyFrame,
   width=\textwidth,
   height=fit,
   location=left,
   style=italic]

\defineoverlay
  [MyFrame]
  [\useMPgraphic{MyFrame}]

\definelabel
  [Définition]
  [headstyle=\bf\smallcaps]

\startuseMPgraphic{MyFrame}
    path b ;
    picture p ;
    p := textext.rt("\white\Définition") ;
    p := p shifted (2BodyFontSize,OverlayHeight-ypart center p+.25ExHeight)
;
    b := boundingbox p leftenlarged .5EmWidth rightenlarged .5EmWidth ;
    fill OverlayBox withcolor \MPcolor{MyColorA} ;
    fill b withcolor \MPcolor{MyColorC} ;
    draw p ;
\stopuseMPgraphic


\starttext

\startchapter
    [title={Chapter one}]

\startMyDefinition

\input knuth
\stopMyDefinition

\stopchapter