Stanley Kubrick's work has inspired many in the cinematic world, I assume Lynch is no exception.

the blue box (or blue cube) in Mulholland drive could hint for Kubrick.
if so, the blue box in the film could represent the film itself that we're watching.

when betty unpacks her blue suitcase (blue box) and puts her cloths in the drawers, she is metaphorically (or is it literally?) "unpacking" her desires and inner self to rita.



when betty lands (lands from above) in L.A, a blue sky city, she is welcomed by a big blue banner (blue box), she is literally descending from above down towards the blue welcome banner (lands from above).



when betty exits LAX and enters L.A, betty is entering the blue box.
betty is accompanied by the elderly woman and her two blue suitcases (blue boxes) in the background are being carried for her (miscarriage?) and hidden in the back trunk of her cab without her knowing (consent).





every time we see the blue box in the movie, it reflects the scene itself in the movie that we, as an audience, are watching.
whatever is occuring to the blue box in the scene represents what is happening in the scene itself that we're watching.
the key for understanding the meaning of each scene in the movie is to just simply look at the blue box in the scene (look at the movie).

notice the blue sign (blue box) in the background of the cab driver, it has names of countries (destinations) written on it with a yellow arrow (yellow taxi) on the top left of its frame, pointing to the left. this sign is literally describing the scene that we're watching - (betty is) leaving the airport with a yellow taxi.