I feel bad because I am going to shred this deck. But, hopefully, you'll see my reasoning and appreciate U/B Zombie mill from another perspective
First off, I LOVE this deck concept. I am completely enamored with Undead Alchemist and Endless Ranks of the Dead. Wow, I would not be across the table from that!
So, looking at your deck I see some things that concern me. First of them is Endless Ranks of the Dead; it creatures zombies during your upkeep based on the number of zombies you have in play rounded down. That's an important distinction because you only have 8 zombie cards in the entire deck. All other zombies you can create have conditions that you aren't always going to be able to meet (e.g. having other zombies already or your opponents having creature cards on the bottom of their library).
If I were your opponent I would let you cast all the Endless Ranks of the Dead you wanted and just kill the handful of zombie creatures you cast. You'd probably have a rough time putting together a legitimate threat in that case.
My recommendation is that you drop Deranged Assistant, Onyx Mage and Vault Skirge for zombies to help the cause.
My next concern are the ancillary cards like Runic Repetition , Grasp of Phantoms and Graveyard Shovel. They don't have any real synergy with your two win conditions which is either "Zombie Mill" (Undead Alchemist and some zombies) or "So many Zombees!" (Endless Ranks of the Dead without Alchemist).
So, here is a link to all the U/B zombie cards in Standard right now. http://deckbox.org/games/mtg/cards?f=47
!b31!793.2
From that list I like Stitched Drake and Cemetery Reaper for your build. The reaper works very well with all of your strategies; the drake is a little concerning because it needs to be "fed" and you might not always have something to feed it. Maybe more on that later...
I also like Gravedigger, Geth, Lord of the Vault and Skinrender for the deck.
Lastly I would like to see the deck get some draw along with a way to feed the Gravedigger and/or Stitched Drake so I think you shoudl run a full compliment of Forbidden Alchemy.
So, the final list could look something like this. http://deckbox.org/sets/100944
I look forward to hearing your thoughts on my suggestions.