it took me a while to track this down. It seems that if one uses \in[…] and \footnote{…} in the same paragraph the following \noindentation-command is ignored. Here is an example:
\setupindenting[big,yes]
\setupdelimitedtext[blockquote][before=\noindentation]
\starttext \showframe[text][text]
\startbuffer
\startblockquote
Is this paragraph indented? It should not!
Is this paragraph indented? It should not!
\stopblockquote
\stopbuffer
Test \in[someref] test\footnote{footnote}. \getbuffer
Test test\footnote{footnote}. \getbuffer
Test \in[someref] test. \getbuffer
\stoptext