We can load the model by pointing using the ID to an that specifies the src to a file:
<a-scene>
<a-assets>
<!-- At first we load skeletal animation blending JSON as asset -->
<a-asset-item id="hand" src="/path/to/hand.json"></a-asset-item>
</a-assets>
<!-- Now we can use that asset with blend-model-->
<a-entity blend-model="#hand"></a-entity>
</a-scene>