If you're trying to print heavy ink deposits onto an average inkjet medium you may well have problems with inks bleeding into one another. It may be that you have switched to a cheaper third party paper or that you are trying to get a very saturated image, for backlit film for instance.
Firstly, profiles from manufacturers tend to have heavy deposits and black is made up from cyan, magenta, yellow and black. Black may be a minority component of the 'black'. Why should this be? (hint - check your ink bill).
The ideal situation is to get a custom profile created for that medium, that should take into account the total ink that the medium is capable of holding before bleeding and give you better colours.
But for those of you who won't do that :-
Black is a typical colour to bleed because it is very visible and has a higher ink deposit than other colours. It is therefore desirable that the black be lowered to the point at which it is acceptable as a black (rather than grey). You may well discover that you can get a quite acceptable black with much less ink. It will save money and help avoid bleeding.
A RIP will often allow a drying time between passes, this can help avoid the bleeding which is caused by 'wet-on-wet' printing. Perhaps I should have added that to my article on 'Why you should use a RIP'.
A rather cunning ploy is to add a keyline between the colours. If the problem colour is black bleeding into red ~(black might be 300% ink i.e. averaging 75% of C,M,Y & K whilst red is 200% magenta and yellow) add a keyline between the colours, it only has to be a couple of points, and colour it grey with a total ink of 100%. That forms a sort of moat that the black and red can both spread into, but you won't be able to see it, honest.
