The Sun in His Splendour is a common heraldic device, not restricted to just the KotBS (blazing squad knights lol). In fact the Sun in Splendour features on both Averland and Wissenland emblems representing the fact they stole bits of Solland - Solland itself noted as having the Sun featuring prominently on it's colours.
So, in short, it's the Battle Standard of Solland (or at least as I see it)
Sun in Splendour
Golden crowns
Knight slaying evil monster
Black field
Solland, not The blazing squad.