You can create a model for the page you’re querying, and do whatever you like in that model (fetching and sorting things). Then you can use that in the querylanguage in the blueprint.
E.g.: model with e.g. myAlbums and myFirstAlbum method -> page.myFirstAlbum.
I don’t think this is possible, only a few fields actually support query language for the default option, it seems. It works for the textarea field, for example.
@bvdputte Have you used this successfully somewhere?
@Cris Default only works at page creation, that’s right.
If the field is required, you could probably use the API query option to force the first child to the beginning. It would then be auto-selected, I think. (not tested)
On a side note: Interestingly, providing a default for a textarea field inside a structure works: