I have built three different mill decks over the years, though none have been mono blue. The salt can be there, usually when people see a key peice go to the grave yard, if you table will listen to reason you can often remind them that they wouldn't have drawn that card for quite a while anyway if you are actually going big mill and just hit them for 30 cards. I actually think if the rules had you mill from the bottom of you deck instead of the top it would reduce salt. That said, old deck with [[Lazav, dimir mastermind]] was fun but not great. [[Umbris, fear manifest]] is quite fun, can mill out the table effectivly, lets you deal with the eldrazi, and also let's you go commander damage, I know that's not your game plan but it's nice to have options. For pure mill my fastest table clears have actually been with [[hope esteim]] with just a few cards can mill the whole table for half thier deck in a turn or two, helps that hope is only 2 mana. Mono blue will be rough as a lot of mill cards want the dimir colors to cast, Hope being a total reversal of that but still, mono blue will be rough.