Xerox phaser 4510 totally black prints. Changed drum cartridge,transfer roller,power supply. Laser unit works.

SOURCE: Xerox Phaser 6250 started printing bad

That really sounds like the imaging unit cleaner has an issue.
If you have one try the imaging unit, I really suspect that is your problem.
Black is Faded or Missing Step 1 Clean the Laser Windows and Remove Excess Toner From the Imaging Unit Step 2 Check or Replace the Imaging Unit Gear Retaining Pin Step 3 Replace the Imaging Unit (installation instructions)
See if this link works. Xerox is infameous for these not working.
http://office.support.xerox.com/SRVS/CGI-BIN/WEBCGI.cgi/,/?St=842,E=0000000000019251353,K=6024,Sxi=13,Case=obj(19377)

SOURCE: Xerox Phaser 6250DP sensor not cleaning

Is there a specific code coming up? That can make it easier to pinpoint the exact problem.

SOURCE: Xerox Phaser 7300DN Laser - Transfer Unit issue

Hold down Back and Information and turn on the printer. You enter in service mode. Then you can reset CRU counters.

Good luck!

SOURCE: phaser 6250 transfer roller

Hi clwest: I hope that this will help you.

Transfer Roller is at End of Life
■ Print the Supplies Usage page to verify the remaining life and Transfer
Roller install date.
■ Cycle printer power.
■ If the problem persists, follow the procedure below
Note
Look at the clear plastic window on the Transfer Roller. If toner is present in the
center of the window the Transfer Roller is at end of life and needs to be
replaced. If no toner is visible, try cleaning the CTD (ADC) Sensor.
The “Transfer Roller is at End of Life” is a warning. “Replace Transfer Roller” is
a hard stop.

SOURCE: Xerox 6250dp

I had this problem when I used a generic toner. I installed a xerox cyan toner, then printed many sheets of a large blue rectangle. After about 15 pages the blue started to spread into the shape, after 30 pages the blue was back. Printing a lot of blue was necessary to get the new toner thru the system.