I was under the impression this was Boletus edulis (or maybe B. aereus, given that it was under tanoak), but now I'm starting to think that "dirtiness" on teh stem is actually the "scabers" of Leccinum. This isn't the best pic, but you can sort of see how they are elevated on the sides. I didn't notice any particular staining when I was cutting it (thought you can see some blue-grey blush here), it did turn very dark after drying.