The MasterBlock controller is not creatable by MAXScript. It can only be created in the Block Control node in the Global Tracks node of Track View. If multiple MasterBlocks are present, you will need to access the MasterBlocks as a subAnim of trackviewnodes.global_tracks.block_control . This is because all MasterBlocks have the same name (MasterBlock).
Once a track has been added to the MasterBlock, a Block controller is added to the MasterBlock, and the track’s controller is added as an input controller to the Block controller. The name assigned to the Block controller is the name specified for the block. The Block controller is then available as a property of the MasterBlock, and the track’s controller is available as a property of the Block controller. In addition, the track’s controller is replaced with a list controller and the track’s original controller is placed in the list controller along with a Slave controller. This Slave controller is controlled by the Block controller. For example, if a block with name "BoxHeight" is present, and contains the height controller for a box object, the block and its track can be accessed as follows:
mb=trackviewnodes.global_tracks.block_control SubAnim:MasterBlock getpropnames mb #(#Blend, #height) h=mb.height SubAnim:height h.object Controller:Block getpropnames h #(#Box01_Height) h.box01_height 157.083-- the box’s height h.box01_height.controller Controller:Bezier_Float-- the box’s height controller