"Clear as mud?" is one of many Oklahoman expressions. Possibly started by the muddy water, yes, but meaning..." I know it's not very clear/you may not understand this well, but do you at least get the gist of what I'm telling you?"
Obviously, mud is not clear, that's the point of the oxymoron. There a mixed parts which make something 'mud', but even while you can't clearly see through muddy water or seperate it's components easily, you can still get an idea of it, at least in part, of what it is you're looking at or for.