HP print problem

i have a hewlwett packard t45 all in one fax copier scannr machine that has worked well for three years until now. the problem being , when i copy something the printer isn't using the black cartridge its using the color cartridge and the copy is coming out a dark blue. this only occurs when copying, not when recieving a fax or printing off the 'net. when i do recieve a fax or print from word or the 'net everything is fine. any ideas anyone?