With last beta: https://dl.dropboxusercontent.com/u/13064071/plantuml.jar
You can have:
@startuml
skinParam CaptionFontSize 10
skinParam CaptionFontColor blue
class AA
caption This is caption
@enduml
The default font size is 14 (default for title is 16, but it looks too much for caption). Do you agree ?
Few tests have been done, so feedback is welcome.