Hi DFS,
Thanks for your message, and welcome to deSEC! 
Each update request can only set the IP address(es) for one host. However, you can create CNAME records for the other hosts which point to the one which has the IP address(es). This way, the IP address(es) will automatically be applicable for the other hosts too (think of CNAME as an alias on DNS level).
In your example, you can create a CNAME record for the mail subdomain, and the value (target) of the record would be blabla.com..
To set up these records, you can either use our web interface or our API.
Stay secure,
Peter