It could be a male or female. Both will roost in boxes (though they both usually prefer smaller cavities as they can warm up the air around them more quickly - a box has quite a large airspace inside, and isn't as cosy at their usual prefered roost sites). The female may roost in the box that she will later nest in, but usually they try and avoid this because they build up droppings in their roost sites, and naturally they want a clean nest site that will not attract predators (smell) or disease. But sometimes they don't have much choice, especially in suburban gardens where there aren't many natural cavities for them.
It can be very difficult to sex blue tits. Trying to sex a lone bird is unreliable, and features such as width of head band and blueness of cap/wings are only really useful when you compare a pair at the nest, when you know you have a male and female. First years are even more tricky, as they're not as bright. And suburban birds are another problem - they're duller than country birds. So it makes it too difficult to tell, really. Many ringers insist that you can sex blue tits fairly reliably. But I think that's maybe an overinterpretation of the criteria in the literature. I have been assured by someone who has compared many hundreds of pairs that it is very difficult indeed.