I don't think having a blademail should turn you off getting a halberd. A halberd provides a ton of utility and tank, which is generally what you need on a lot of type of heroes that build a blademail, on top of that those two items really force the enemy carry into buying an early BKB if you get your items quickly. Forcing the enemy #1 position to alter his item build around what is presumably a #3 position hero is really beneficial to your entire team.
Additionally, forcing the enemy carry into an early BKB means he will have lower charges later in the game, and these days you can't just buy a new BKB, so it kinda messes them up. Long story short halberd is a great item.