Sight is a strong part of it. Sound is the other part.
That rachety low sound that the Bewilderbeast was always making? Notice how the volume on it went up when he was exerting dominance? Any dragon that could hear that was in danger of its influence. I may be wrong, but I would also swear that when Hiccup put the blindfold on Toothless, he also took care to cover his ears with it (although that may not have lasted long, the way they were flying).
For bonus points, you could hear a very similar sound being put out when the Vikings landed at the dragon nest in the first movie.
It seems like they've done a lot of work in linking actual animal behaviors to these mythical dragons. The direct control thing may not be found in nature, just like dragons aren't, but just about every other element is. Alpha dominance in many animal packs is a combination of factors, so sight and sound may not even be the only two.