Hi Markus,
Maybe it could be more efficient to do some API requests to find the right entities rather than filtering many alternative labels as part of the query. It's not a pattern that we should encourage for production ;-)
I don't think it would be more efficient to do any api request before querying blazegraph because the api is way slower than the triple search in BlazeGraph. For exposing SPARQL for newcomers it would perhaps be nicer to add the actual Q/P ids instead of huge lists of UNIONs but for the internal query I guess passing everything to BlazeGraph and let it do the right things with it is imo better and more efficient.
Best regards Bene