It very much depends on at what stage of development a partner is lost. Blue tits usually have large clutches so keeping warm shouldn't be too much of a problem after 7 or so days. The size of the clutch works against them when one partner is lost.
There is no way one parent can keep up feeding 9 or 10 chicks, so often the chicks are abandoned and a new partner is sought so the season is not wasted. If the chicks are only a few days from fledging the remaining parent may try to raise as many as it can, but some of the smallest ones will probably expire.
You need to remember that, although they are wild animals, each individual may vary in their behaviour so there are no standard rigid rules in these situations.