It is little secret that Space Marines are, both in design and function, inspired by Medieval knights. Even just a passing glance at some of their armor design clearly shows this influence.
There are certainly other looks often compounded upon it, such as the Ultramarines Romanesque aesthetics and the Space Wolves viking one, but still a medieval knightly appearance remains in some regard. The two chapters that both equally exemplify this are the Dark Angels and the Black Templar, this made doubly so as each is expressly inspired by some of the most famous knightly orders, the Knights Templar, the Knights Hospitaller, and the Teutonic Knights.
Against what would originally be thought, the Black Templar are, in fact, not inspired by the Knights Templar that honor lies entirely with the Dark Angles. The Templar influence, that of a monastic order turned militant, is well displayed with the large large quantities of monastic robes the Dark Angels can be seen sporting. While the Dark Angels don’t sport the traditional Templar cross and their white and green coloration differs from the historical Templar’s white and red there are a wonderful amount of Dark Angels lore that connects them to the Templars. The Dark Angles were the first Space Marine legion, just as the Templars were the first Catholic Military Order. The Knights Templar headquarters during their founding and for most of their history was inside the Dome of the Rock while the Dark Angels center of operations is a massive star ship simply known as “The Rock.” While it is more myth than truth, the Knights Templars are known, and have been known for hundreds of years, in pop culture as secretive to the extent that significant conspiracy theories have developed about them. In the same vein, all descriptions of the Dark Angles have them as exceptionally closed off and with secrets so serious only the inner circle of their own organization can know of them. Any outsider who finds out, even if they are technically on the same side, is killed. This leads to the final point where the Dark Angles, or atleast parts of them, are heretics and traitors to the order the purport to follow. It was similarly decrees of heresy and branding the Templars as such that saw the disbanding of the order and the burning of several Templars at the stake.
This leave the Black Templars with the Knights Hospitaller and the Teutonic Knights as their inspiration. The Black Templars take less from the Hospitallers, primarily being black as the color of their armor as the Hospitallers wore black robes and surcoats. The Black Templars also take their chapter symbol, the eight pointed cross, from the Maltese Cross that originated with the Knights Hospitaller.
This is then mixed with heraldry of the Teutonic Knights, they share the same black cross on white background that the Black Templars have in imitation. While the armor of the Space Marines are black, the robes they wear over it are white with black crosses on them. The concept of offensive crusading, which in the Warhammer 40,000 universe the Black Templars exemplify as they pursue a 10,000 year long crusade to conquer planets, is also singularly inspired by the Teutonic Knights. The Knights Templars and Knights Hospitallers both function more as defensive orders, defending Frankish conquests in the Levant after the First Crusade, while the Teutonic Knights set about capturing the area that would later become Prussia and transforming it into their own state.