Absolutely nothing. This is just a belief which has come about, the reason people select a pen and use it only for their spells is because it gives that pen an importance, a specific use, and in one sense 'consecrates' it to that use.
It is really a psychological effect at first, until you use it over and over and it then may be suited to that purpose, but there will be no detrimental effects if someone used it. You may wish to simply cleanse it and rededicate it.
I personally have never blessed a pen, sometimes I need to write things for rituals, any old pen does the trick for me!