As I understand it, a fiendish servant gets precisely what the Blackguard class description says it gets, based on your Blackguard level...much the same as a paladin's mount.
Even though its HD may increase, no changes are made to it beyond those specified in the class description. Thus it doesn't increase in size (with attendent increases in stats and so on). See also the druid's animal companion for related rulings.
Edit: Increases in HD -do- lead to possible new feats (1 per 3 HD) and stat increases (+1 to a single stat per 4 HD) and skill points (by creature type modified by it's Int score).
Even though its HD may increase, no changes are made to it beyond those specified in the class description. Thus it doesn't increase in size (with attendent increases in stats and so on). See also the druid's animal companion for related rulings.
Edit: Increases in HD -do- lead to possible new feats (1 per 3 HD) and stat increases (+1 to a single stat per 4 HD) and skill points (by creature type modified by it's Int score).