You can kinda do it either way.
Blackguard is a base D&D PrC and therefore in its original form isn't explicitly tweaked for the Forgotten Realms setting, where the Planes are generally less important than the Gods who dwell there. As I believe BG is a Divine class on Arelith, you are likely to be serving a God/s and your fiendish support would be an agent of your Patron.
Gods are served by all manner of fiends, angels, monsters, and creatures. The way it works on Arelith trends somewhat toward the "anti-paladin" path, but it still allows scope for Blackguards that are free agents striking bargains with (or enslaving) fiends in service to himself, or a demon lord, or other weirder entities, similar to a Warlock.
In all honesty you can go either way and you will be absolutely fine. This includes being a Blackguard of a Neutral Deity, which is often very interesting. A BG of Helm or Red Knight will be rather different to their fellows-in-faith.