It is actually a real blue one painted just for the movie. It shows up in multiple clips in the preview with different background
No, I think it looked somewhat blue due to the highly saturated (and manipulated) cinema color profile of the film. In this behind-the-scenes video about the movie, which is shot in normal TV color profile, none of the J-10s looked blue; they were all in normal PLAAF gray. Besides, if the moviemaker really wanted a blue J-10, it'd be easier and likely cheaper to digitally paint it blue in post-processing.
If you have proof that they painted one blue in real life, then please provide it. We are all open minded here.