John,
In the next release of the extlib, the mail service will include support for the JSON “bcc” property. The “bcc” property is an array of objects, just like “to” and “cc”. When you create a new message with a “bcc” property, the mail service will map that to the BlindCopyTo item. When you read an existing message with a “BlindCopyTo” item, the mail service will map that to the “bcc” property. By the way, the mail service will also support the MIME “Bcc” header field – both as input and output.
I can't say for sure when we will post a new release of the extlib. Hopefully, that will be in a few weeks. Meanwhile, you could try modifying the code yourself. JSON message handling is implemented in the com.ibm.domino.commons bundle. The most relevant classes are JsonMessageAdapter and JsonMessageParser.
Thanks.
– Dave