I dont know anything about Wahammer 40K but I have bought all of the Black Templars so far. I'm slowly learning the lore from what I understand these Imperial Fists predate the Templars so they would fit in with them atleast in my head canon.
Yes, Black Templars were created from the Imperial Fists, you could consider them brother chapters. Rogal Dorn, the primarch of the Imperial Fists and Black Templars(and other imperial fist successor chapters) has an order called the Last Wall Protocol, where all sons of Dorn chapters become a legion again. So it makes perfect sense for IFs and BTs to be displayed together if that’s your head cannon. Plus it HAS happened before IIRC. Altough it’s worth mentioning these are 30K imperial fists, they predate the Black Templar(at least the primaris we have as joytoy form) by around 10K years.
And as suck from the previous comments, the black templar's very likely had these terminators at the beginning before they got around to painting them in their new colors
