With a web search I find different answers. The ones say it is a BIOS setting (what exactly?) and the others I would say would have to make sudo grubby --update-kernel=ALL --args="bgrt_disable".
Supposing the recommendation for adding the above mentioned kernel argument is valid[1], on Silverblue one can modify kernel arguments with the kargs subcommand of the rpm-ostree command. E.g.:
rpm-ostree kargs --append=bgrt_disable
That would create a new deployment, which would have the appended argument included.