How Ty Lee learned chi-blocking?
Yes, I know about all of the of airbender ancestor theory or she is an airbender herself, but I don't like it, I think it's stupid. This ability is ancient one, part of the history, was known in all four nations, airbending is not related. Maybe they invented it long time ago, but regardless, it's not related. The water tribe learned about chi in healings and all of that so they might have invented it themselves. We do not know. So yes Ty Lee has grey eyes, who cares. The eyes are the only hint for a possible ancestor, which could be logical because we know from avatar legends that's a lot of airbenders lived in the Fire nation in Roku's era. But it's still not an explanation for her abilities.
What I want to talk about is how she learned this secret technique. We saw in the Avatar comics chi-blockers from the Water Tribe and the Earth Kingdom, but only now in the roleplaying game we got information about chi-blockers besides Ty Lee.
Fai Lan was a gang member in the Fire's nation capital city who knew chi-blocking. He was active few years before the series begin. Now a little reminder, Ty Lee had a hard time with her sisters, it wasn't easy for her in her house. So, she could have meet him at some point in the street of the city, even though it's not a place for a nobleman daughter to be, Ty Lee doesn't seem one who care. So, she could learn it from him, even before the circus she was good in acrobatics, so he could be impressed by her.
I know it can be sound weird that a criminal will notice Ty Lee and won't hurt her. So I have another direction but it less solid. There was another person from the Fire Nation who knew chi-blocking, it was Zeisan, Sozin's sister. Maybe she taught the technique to someone and this person was there to teach Ty Lee or someone else this person taught the chi-blocking. If Ty Lee's knowledge indirectly related to Sozin's sister it could be really interesting and ironic.