All shard blades (that we know of) fit in these catagories,
A: Investure; All Shardblades are invested, Either by virtue of being a god metal or by being invested by an outside magic system EX: breaths, Tanvastium (Maybe Ferchemy? Probably needs some weird abuse of the system.)
B: Intent; All Shardblades have some form of intent, either by a Bio-Chromatic command or by the intent of their investiture EX: Destroy evil, The Aon in Seons, Spren's whole thing, Presumably the Honorblades.
C: Sentient/Aware; All known Shardblades are self-aware to some point; some have memory issues while others refuse or can not talk (Brandon has said the Honorblades are sentient), but they all have the capability to think.
Remember just because a blade is invested and has intent, does not mean it is a Shardblade. Correct me if I'm wrong, but the Raysium dagger was never referred to as a Shardblade right?