If you set QgsBlendModeComboBox to extend QComboBox instead of QWidget, a basic combo box can be used in the layout. As it is, there appears to be no combo box (or any control at all) in Qt Designer (snap from previous layout):
![hidden-control](https://f.cloud.github.com/assets/1295919/293705/c70550b8-93a3-11e2-929a-adecd9e15f4a.jpg)