I was wondering if Legacy BIOS is the blue interface, but then why is EFI an interface (UEFI) and a boot mode in one while BIOS is the interface and Legacy is the boot mode (separate things)?
Is (U)EFI working as a boot mode itself and as an interface?
Both BIOS and UEFI are firmware types (or boot modes, if you want).
Neither of them corresponds to any specific visual interface. It's just that graphical setup screens have become more common with UEFI-based firmwares, as the architecture of UEFI makes it somewhat easier (and modern motherboards have more flash memory to fit all of it, too).
But what kind of setup screen you get is entirely the manufacturer's decision – a blue text screen doesn't mean BIOS and a graphical screen doesn't mean UEFI. (Some BIOS-based PCs have had graphical setup screens a decade ago.)
