THey will have started nesting last month- eggs may have hatched by now or due to. If your nest box is being used you should see lots of activity- when feeding young blue tits make umpteen visits to the nest to feed young- I have watched them make visits at less than one minute intervals!!
So if you are not seeing activity it may not be in use. Wait til Autumn ( I think the actual date is somewhere on the RSPB Site) before inspecting the box and cleaning it out.
It is very likely that blue tits will raise a second brood one the first has fledged