Have you tried changing the switch position on the cartridge? All-in-all, you should be able to boot X-Flash without the cartridge in on your debugger, plug in the cartridge and press R1 and then R2 enable comms and force a detection of the cartridge. From there you can flash CAETLA 0.34 and then retry again afterwards flashing UniROM.
It's very strange. It shouldn't make the screen go blank, and if it is, it's throwing junk on the bus when it's plugged in (unless somehow it's shorted and holding the PSX in 'reset', or the reset line is being held low since there is a reset line on the parallel port). I'd try taking the Game Shark apart and lifting the chip select pin and shorting it to ground (or VCC depending on the EEPROM's specifications). That way, the cartridge is on the bus, but the chip is disabled. Then, when X-Flash has booted, re-enable chip select. Otherwise, the EEPROM will need to be manually reset using an EEPROM programmer.
Development Console: SCPH-5502 with 8MB RAM, MM3 Modchip, PAL 60 Colour Modification (for NTSC), PSIO Switch Board, DB-9 breakout headers for both RGB and Serial output and an Xplorer with CAETLA 0.34.
PlayStation Development PC: Windows 98 SE, Pentium 3 at 400MHz, 128MB SDRAM, DTL-H2000, DTL-H2010, DTL-H201A, DTL-S2020 (with 4GB SCSI-2 HDD), 21" Sony G420, CD-R burner, 3.25" and 5.25" Floppy Diskette Drives, ZIP 100 Diskette Drive and an IBM Model M keyboard.