Hi,

There is a tutorial how to use the STM32 Discovery boards as Back Magic 
probes, and one section refers to why the DFU does not work on the 
STM32F429I board.

http://embdev.net/articles/STM_Discovery_as_Black_Magic_Probe#F429Discovery_board_doesn.27t_enter_System_DFU_bootloader

I believe the explanation is wrong, and the reason it does not work on 
the STM32F429I discovery board is that the USB OTG wired out is the USB 
OTG2 peripheral in FS mode. DFU supports the only the OTG1 peripheral, 
see STM32 AN2606, section 18.1.1, table 38. PA11, PA12 pins used only.

Table 40 also shows only OTG1 peripheral for bootloader v9.x

There is a "discussion" page you could post your findings to, so others 
are more likely to find it:

http://embdev.net/wikisoftware_en/index.php?title=Talk:STM_Discovery_as_Black_Magic_Probe&action=edit&redlink=1