Quick guess, try running the property value through blenc first.
blenc "property value" | grep ^BLencrypt: | cut -f2 -d:
That looks nice!
Where are all these things documented? I'm sure I could do a man on this, but couldn't see it in the standard pdf docs.
I do not know where the documentation for blenc is, I created ticket 10938 requesting blenc be documented.
The following will set the value for the encrypted property TEST_ENC_PROP to property_value for server1:
KEY=`blcli Server getServerDBKeyByName server1`
VAL=`blenc "property_value" | grep ^BLencrypt: | cut -f2 -d: | tr-d '[:cntrl:]' `
blcli Server setPropertyValue $KEY TEST_ENC_PROP $VAL
It's all documented in the "BLCLI Help" API.