Yeah but I mean, Blackbeard is rather Luffy's Mihawk. He's his rival, yes, but Blackbeard is so much ahead of Luffy, bringing him up as an equation is kinda faulty tbh.
Meanwhile, the gap between Kidd and Luffy is much closer than the gap between Luffy and Blackbeard.
Yet again, it's about your point of view whom you consider Luffy's true rival. And even if Blackbeard is Luffy's 'true' rival, we can say at least that Kidd is a much better rival to Luffy than the other supernovas, including Law.