I lived and traveled in Thailand for 10+ years. Obviously, no major problems, or I wouldn't keep coming. There probably are some Thais who are racists toward Blacks, but it has not been a major issue. The very few problems I've had have been with non-Thais. I've experienced some extra scrutiny with security, police, immigration. But as soon as they see the US Passport, complete change in demeanor. "Oh! You're an American." Smiles and waived through. It's like the Mexican - American comedian Fluffy joked. "I had to travel 10,000 miles to be treated like an American."
The question is: Are you just trying to get a DTV visa? Or are you really going to train Muay Thai? If you are not really interested in training Muay Thai, find a cheaper solution. If you want to train in one best, World known Muay Thai gyms for six months, then 40,000 BAht is a bargain.
Easiest and cost free. Put your cell phone on Wifi calling. Connect to a reliable Wifi network. Can send and receive calls as if you are in the USA. Been doing this for years from several different countries.
Book your flights to BKK from Chicago or Houston, usually $800 - $1000 round trip, due Eva Air and other airlines matching. Book your flight to Chicago or Houston separately, for me from ATL usually around $200 or less RT. Newark used to be cheap also.
If you are over 50 you can get a retirement visa. Not permanent, but the best offered. Next best is a Destination Thailand Visa, good for 180 days stay, multi-entry, last for 5 years.