Is LoH: Trails of Blue part of the Zero no Kiseki as well as the next one we're getting?
Yes, it is. Trails of Blue is the sequel of Zero no Kiseki also know as "Ao no Kiseki" in Japan. And the next game we are getting is "Sora no Kiseki Second Chapter".
If so, has the sequel to the game we have, not been released yet in Japan either?
It was released in 2006. The sequel they are getting in Japan right now is Ao no Kiseki.
This might help you to understand things better:
(2004) The Legend of Heroes: Sora no Kiseki First Chapter (FC) -- Known as "Trails in the Sky" in America
(2006) The Legend of Heroes: Sora no Kiseki Second Chapter (SC)
(2007) The Legend of Heroes: Sora no Kiseki the 3rd
(2010) The Legend of Heroes: Zero no Kiseki
(2011) The Legend of Heroes: Ao no Kiseki
As you can see, Xseed has a lot of work to do with this series.