Hmm, I have never bricked my cappy (knock on wood, lol) and I have never messed with jtag. But I have read on this forum that you can make a simple jig with three one hundred ohm resistors that will boot the phone into download mode. Again, I haven't bricked my phone, so I am not sure if this would work with a black screen bricked phone, but it seems simple enough, and cheap enough, to test. Basically, you take three 100ohm resistots and wire them together. Then, you touch the leads to pin 4&5 of the micro usb connector. This is supposed to boot the phone into download mode. From there, you can hook your phone to your computer via usb and use odin or a similar tool to flash back to stock. Again, I am fairly new and have never tested this personally, so I don't know if it works at all, or if it will work with a black screen bricked phone. I hope this does help you out, though!
Sent from my Samsung Captivate using XDA App.
ROM: Cognition v4.1.1
Kernel: eXistZ's KB1 #5 Voodoo Sound v4, Voodoo Color, Voodoo lagfix
Modem: I897 UCJJ4 w/ HSUPA/HSDPA enabled