With MkIV blocks are hidden by default while MkII prints them, add \keepblocks[Block]
to your document and the blocks will appear.
Using \keepblocks worked thanks.
Unfortunately, now the \reset command doesn't seem to work in mkiv (see attachment), but's ok in mkii.
Glen
-------
\defineblock[Block]
\defineenumeration[Enum][location=left,text=Test]
\setupblock[Block][before=\startEnum,after=\stopEnum]
\keepblocks[Block]
\starttext
\section{Use}
\useblocks[Block]
\reset[Enum]
\section{Define}
\beginBlock
jhdsa hdsfhj lh dsfjh jkhasdf
sdakjhfsal sdkajhf kljsdh
\endBlock
\beginBlock
jhdsa hdsfhj lh dsfjh jkhasdf
sdakjhfsal sdkajhf kljsdh fhlkjsad kah dkjh akjhd flkjs
lkjsad flkjh salkjhf asd
\endBlock
\stoptext