Updated: Jul 29, 2020
I was so happy when cryptomatte was ported to blender, what a time saver! But after the first test render and importing the multilayered exr from blender into nuke a huge bummer, it was not working nothing appeared in the Layer Selection of the cryptomatte node. No mattes to work with!?
What was i doing wrong? Took me two days to find the solution after reading about similar issues across different cg blog and forums. Funny that i found the solution on a Vray blog post about a similar issue from a 3dsMax render in nuke.
So here is the solution.
Connect a ModifyMetaData to the read node and click on the + button to add a new set action.
Double click on the empty key section and choose exr/cryptomatte/#######/name.
With the ViewMetaData node you see what the value is of the key section and you just have to change View Layer.CryptoObject to View_Layer.CryptoObject to the get to see the crypomattes in the Layer Selection of the cryptomatte node. Meaning replacing the empty space by an underscore, it was that easy.
Repeat this for CryptoAsset and CryptoMaterial if needed.
Cheers! Now you can use the cryptomatte channels from blender cycles in Nuke.