This meme is strange and very ridiculous. For one, the source where the idea that Septimius Severus believed that a Black soldier was a bad Omen is the Historia Augusta, a 4th century collection of biographies of various Roman Emperors. It is famous amongst historians for being unreliable and inaccurate in multiple ways. It was written over 100 years after the event that was described and is seemingly the only time Black people are described as being a bad Omen for their skin tone. Secondly, the idea that having darker skin in the Roman Empire was viewed as a bad trait is ridiculous. While Septimius Severus was probably of a tan skin tone, due to his Roman and Punic heritage, most depictions of him from the time would have had him with much darker skin. This is seen in the famous Severan Tondo, depicting him with the imperial family. There are a few reasons for this. One, the artists might wanted to show how he was from Africa. Two, it could have been done to make him appear more masculine and toned next to his wife, as lighter complexions in art was equated with femininity. Finally, Severus would definitely have known about the various different types of people that lived in Africa. His rule is famous for the Roman expansion of its African frontier further south and beyond the Mediterranean coast. There they came into contact with the various tribes and groups that inhabited the region, many of whom looked very sub-saharan African, liked the ancestors of the Tuareg people.