Unfortunately the lovely people at Epson make it so that you have to replace the blue cartridge even if you are only using black.... sorry!
If you are looking for cheap replacements, check out Selectafont, they have a range of compatibles which are cheaper than using Epsons Genuine cartridges. Just ordered a set of their T0711/712/713/714 for my parents printer - was less than £8.00 for all 4!!