The reason why you're getting XML inside JSON can be found in documentation of this overload of BrokeredMessage's constructor. Let me quote:
Initializes a new instance of the BrokeredMessage class from a given object by using DataContractSerializer with a binary XmlDictionaryWriter.
I also see you're adding values to 'properties' object, but you showed neither declaration nor usage of this object. If it's related to your scenario, show us your complete code (including the population of streamWriter), if it's irrelevant, throw such code away.