Crystal Maiden - Frostfire take you !

Her ultimate is rarely used in-game, as CM would be much more useful disabling people and slowing them down rather than spend 4 seconds doing nothing but standing there and hoping that random blasts hit opponents.

Guinsoo don't give you the Magic Immunity which is very important to put your ultimate without being disabled. The Nova build is viable too and this is why I've put a Nova build too.

Ghost scepter is great too, i'll add it in situational.

My core is quite expensive but i generally get it all times in mid game because my build is based on aggressive playstyle in lane, so you'll get kills fast, and so you have more gold for a more expensive core.

Nova is your main harassment skill and what makes you threatening or annoying in the lane, why would you suggest a build that leaves it at level 1? It's really much better if you max it first together with your Aura and leave Frostbite at level 1 early, the 1 second disable is pretty enough and also to keep mana cost at minimal, it's the generic way to play Crystal Maiden. And oh, I'd rather drop that BKB and grab a Guinsoo, remove Blademail and grab a Ghost Scepter. Frankly your core is quite expensive for a Support, an alternative cheaper course would be to grab Tranquil Boots, an Urn and a Bracer or a Headress, another option is to grab Medallion of Courage and/or a Ring of Basilius for Armor.