It is common for brass to 'tarnish'.  When it tarnishes it takes on a slightly blackened look.  This doesn't affect the funcationality of it at all, but it does affect it visually.  It is possible to restore it using a brass restoring solution like 'Brasso'.  There is no need to do this but if you  decide to restore the slide visually, put a little electrcial/insulation  tape over the anodsied body of the instrument to protect it.  Use a cloth or paper towel, aplly brasso and rub the brass slide to restore it's apearance.  Make sure to clean the slide afterwards with some meths, IPA or a alcohol based cleaner to clean the surface of the brass.