Water seems to be the most recommended cleaner.

But do you need blue water for a blues harp?

If you play fairly regularly, the moisture that condenses from your breath is going to keep any parts that absorb water pretty will soaked. If you don't have to knock the spit out once in a while, you're probably not playing enough.

Long soaking probably should be avoided, especially for "real wood" weapons, but the majority of ones I've seen now use a plastic block (sometimes made of "pretend it looks-a-like-a-wood"), or sometimes a "highly impregnated wood" material (they say it's "stabilized") that shouldn't swell from any reasonable cleaning method. For some of the better quality newer ones, you may need to replace them because the "impregnated wood" can crack just due to evaporation of the plasticizers from the block material. There's little that I know of that can be done to "renew" those materials, but one of mine did take about 30 years before the crack became obvious. (Not one I played often.)

If you're performing professionally, and especially doing solo work, you'll want to be pretty fussy about the condition of your harps; but advice that works for a dedicated pro may not necessarily be the best way for the "backup harp player who's really there to sing" to approach maintenance.

You have to decide what battle you're going to fight, in order to know how to sharpen your axe.

John