Betta Fish: Is this tail color normal?

This is Ricky Bobby. He lives in a 10 gallon tank with some plants and a filter. Tank parameters look fine. Zero nitrates/nitrites or ammonia on test strip. pH and alkalinity ok. Temp 76. I had the tank cycled for a least a month before we got him, had him about a month or so. He was smaller and only red when we brought him home but I’ve noticed this black color developing on the edge of his fin as he grows. I know they can have color change and I hope that’s all it is. It’s not fin rot, is it? Is 76 too cool? He’s very curious and active otherwise, probably the most active betta I’ve had.