Thread safe implementation of circular buffer

i think it looks fine, except that there is some pointless copies of Mat made in send. You don't need the new, you can directly push the argument of send to your cb.

Yes.
If you lock all the public methods with the same lock it will be threadsafe.

You could consider using read-write locks, which may have better performance if you have a lot of concurrent readers.

If you don't have a lot of readers, it will just add overhead, but may be worth checking the option and testing.

Your implementation is similar to the one shown by this blogger. You should read that blog to see if you missed anything in your implementation.

If your Mat objects are expensive to create/copy, you should avoid continuously creating/copying/deleting them. Instead, you should have a pool (aka free list) of Mat objects that continually get recycled in some kind of pipeline architecture. I describe this type of architecture in this answer to a related question.

In that answer, I suggested using a blocking stack to implement the pool, but you could also use your blocking circular_buffer. The reason I suggested a stack was because I thought it may be more cache-friendly, but I never actually measured to see if it would make a difference.