it means cwm has not been flashed properly .so only you get black screen when you enter recovery .
Step 1: Flashing the Custom Recovery UI by XDA.cn
Again, make sure your device is S-OFF.
Again, make sure your memory card is formatted FAT32.
Place the PG76IMG.zip file into your phone SD card.
Disable Fast Boot from your phone settings (Settings > Power > Disable Fastboot)
Turn your phone off.
Hold down the "power" button and the "volume down" button for a few seconds till you see the HBOOT screen.
Don't press anything. The system will scan for some files and it will prompt you for an update. Move the cursor to OK (volume up) to start the flashing process.
After it finishes, press the "power" button to restart the system.
Let Android boot and delete the PG76IMG.zip file from your SD card via USB.
Congrats! Custom recovery has been installed. Process to step 2.
Step 2: Flashing the rooting script by XDA.cn
Copy the root.zip file into your phone SD card.
Turn your phone off.
Boot into recovery: Hold down the "power" button and the "volume down" button for a few seconds till you see the HBOOT screen. Let it do its thing and then select "RECOVERY".
At the recovery menu, use the volume buttons to select "install zip from sd card".
Then select "choose zip from sd card".
Then find the "root.zip" file on the list.
Select "YES".
Let the script do its thing till you see "Install from sd card completed".
Select "Go Back" on the menu till you find the "power off" option and select it.
Turn your phone back on and let Android boot
you can use the custom recovery PG76IMG.img or else clockworkmod recovery from
http://www.clockworkmod.com/rommanager
cwm recovery have more options ,so i prefer you to use that .
you can find those FILES in the source which i have specified.
source :
http://xdaforums.com/show....php?t=1195275