If you see a blue bird, does that make it a Bluebird? Not necessarily.
If you see a bird that isn’t all blue, can it be a Bluebird? Perhaps.
This first image is a Tree Swallow. It’s probably one of the prettiest blue birds we see around the pond (and it’s almost entirely blue). I am asked from time to time if a Tree Swallow is a Bluebird. I’ll reply that its a pretty color, but it’s not an Eastern Bluebird.
To see an Eastern Bluebird, you have to look for orange.
Admittedly, this is a carefully chosen view of an Eastern Bluebird. The back of the Eastern Bluebird is blue, particularly on the males. Nevertheless, I find it amusing that from some perspectives, the Eastern Bluebird doesn’t appear to be blue at all.
On the west coast of North America, there are Western Bluebirds, but they have more orange than Eastern Bluebirds. However, if you go to the Rocky Mountains, you’ll find Mountain Bluebirds, which are completely blue. And they’re Bluebirds.