The Sydney-born Cronulla prop had been barred from representing NSW after playing one Test for New Zealand - a tier one nation - in 2017.
Fonua-Blake has mainly represented Tonga since and had all but accepted his lot before the NRL opened the door in February with new eligibility rules.
Criteria changes meant Fonua-Blake's sole Kiwis Test cap didn't count against him, paving the way for him to be named in Laurie Daley's squad for the May 27 series opener in Sydney
"I thought this game was probably past me," the 30-year-old said.
"I was just a fan from afar watching the games.
"When the post got put out that the rules were changing, it was something that I was wanting to be a part of.
"I'm just happy that I got a call-up and all the hard work sort of paid off."
Fonua-Blake said he hadn't regretted his decision to represent New Zealand early in his career.
He is also joined among NSW selections allowed under the relaxed rules by Penrith outside back Casey McLean (also New Zealand) and Sydney Roosters forward Victor Radley (England).
"When I made my decision when I was younger, I backed that," Fonua-Blake said.
"I feel like they (NSW) backed my decision as well.
"There were always conversations like, 'Oh, do you reckon you would have made it?' or whatever.
"And I just said, 'Oh, look, I don't know if I would have made it, but I would have loved to have given it a shot'.
"Now we don't have to think about the what-ifs. I'm here now."
Fonua-Blake had been sounded out early over a coffee by Daley.
"I told him it would mean the world to me," Fonua-Blake said.
"You grow up watching these games, as a kid, you know, you want to be a part of them.
"He just said, 'Alright, well, go out there and prove to me why you should be in the team', and I feel like I've done that."
The Sharks big man had a message for NRL chair Peter V'landys and chief executive Andrew Abdo.
"Thank you. If I see you guys out, I'll buy you a coffee or a beer," Fonua-Blake joked.
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