Oh i like these ones, though maybe a bonus on craft checks involving blackstone is in order. Something you may also want to consider is give them a natural slam attack (only 1d3 but still). It makes sense, mostly because I see them as being beetle-like and larger than other hivelings while still being classified as "small".

I created a similar race actually that secreted an alchemical enzyme/resin that they used to craft everything, probably why i like this.