Blue Tit using house hole

I blocked a small hole in my house today after some of our cavity insulation had been forced out, believing it had been sucked or blown out. When dusk came i spotted a blue tit trying to enter the hole, to my guilty horror. I unblocked the hole but haven't seen the bird since. I did check the hole first before blocking and there was no sign of feathers, grass or other nesting materials. What would be the best course of action now? Fit a nest box in front of the hole? Allow the bird to continue using the hole?