Shardblades each have a gemstone in the pommel (or at least in some location; largely irrelevant for the present purpose). Fabrials are devices which gain extraordinary abilities via a spren captured in a gemstone fit in the fabrial. It is my contention that shardblades are a sort of god-fabrial. I suspect that rather than a simple spren being captured in the gemstone it is a piece of a Shard voluntarily splintered and placed or invested in the gemstone (or even, it may be the gemstone) for the express purpose of forming the shardblade.
When you say this, it made me think of the fact that shardblades are increasingly heavy so you would need a shardplate to use them effectively. When a shardplate is cracked, it leaks stormlight, which both the armor and the old knights radiant were able to use. If this were the case then wouldn't the old knights radiant be able to draw stormlight from the armor like Kaladin did from the gems thus enhancing their own abilities to wield a shardblade?
(just a thought)