While running this query I noticed a lot of links like "http://www.wikidata.org/.well-known/genid/77a2ecfc46a8a907a59596e7214eca94" pointing to non-existing pages, instead of some marking for "unknown value" aka SomeValue.
I do not know the proper way "unknown value" / SomeValue values should be display as , but probably dead URLs were not intended
See the blank node deprecation announcement for more context on those URIs.
We can improve how they’re displayed, but for the record, previously “unknown value” (blank nodes) were shown as strings like t76842948 (“t” plus some random number), so it’s not like this was much better before the blank node deprecation.
To address the visualization within WDQS (and not the dereferencing): Would a new prefix wdunkownvalue: <http://www.wikidata.org/.well-known/genid/> added to wikibase.queryService.RdfNamespaces.NAMESPACE_SHORTCUTS.Wikidata be a feasible solution? It doesn't sound too complex, and the descriptiveness of the prefix might guide WDQS users to the correct conclusion regarding its meaning, even those not familiar with skolemization and well-known URI as skolem IRIs.
That would probably work, but seems like a hacky and potentially misleading solution to me (misleading because you wouldn’t want to use this prefix in a query). I think it shouldn’t be that much more effort to implement this task “more properly”, it just hasn’t been a priority for the dev team so far. If anyone wants to work on it themselves, the relevant code would probably be in wikibase/queryService/ui/resultBrowser/helper/FormatterHelper.js, formatValue() method, and it would use a new i18n message (pointing to {{msg-mw|Wikibase-snakview-snaktypeselector-somevalue}} and {{msg-mw|Wikibase-snakview-variations-somevalue-label}} in qqq.json).
There's also T173248 about displaying it better (much older, from before they were turned into URIs).
I agree that /.well-known/genid/ URLs should redirect to something instead of giving an error, because even if the query service UI displays them better, people fetching data from the query service will also get these values, and are also likely to open them wondering what they are.
We already do something similar for reference nodes, e.g. if you try to open http://www.wikidata.org/reference/0a99d4aeea2a9c9530f8886075c848033a8aac69 it redirects to https://www.wikidata.org/wiki/Help:Sources. Using https://www.wikidata.org/wiki/Help:Unknown_value should work. It currently redirects to a section on Help:Statements but I would still use Help:Unknown_value in case that ever changes.
It looks like they're defined in https://gerrit.wikimedia.org/r/plugins/gitiles/operations/puppet/+/production/modules/mediawiki/files/apache/sites/wikidata-uris.incl and https://gerrit.wikimedia.org/g/operations/deployment-charts/+/master/charts/mediawiki/templates/lamp/_site_helpers.tpl. I assume copying the /reference/ one and adding a line like this would probably work?
RewriteRule ^/\.well-known/genid/(.*)$ %{ENV:RW_PROTO}://%{SERVER_NAME}/wiki/Help:Unknown_value [R=303,L]