Poorly plum tree

What's your location?
Can you show a picture?
Which variety of plum and which rootstock?

My guess would be that the tree was diseased when it arrived - diseased trees often put on a really spectacular blossom display as a final last gasp at producing some offspring before they collapse and die.
So, in fact, contrary to popular belief, a tree with lots of flowers is actually unhappy or stressed. A tree which produces no flowers but strong growth is very happy.
Young trees should not be blossoming too heavily unless they are a few years old and on dwarf rootstock.
A frost in April really shouldn't kill leaves a plum tree - they're fully hardy (although frost can kill blossoms). Even if it had been frost-killed as it was leafing out (but it would probably require a -10'C frost), if the roots were still alive it ought to be having another attempt at growing back. If it isn't growing back, it again points to a sick tree which was diseased when purchased.

I also notice that due to the wet weather in this area, many of the stone fruits (we have lots of cherries in this area) are looking very sick. Probably the wet weather has allowed canker disease to attack them. Some varieties are more prone to disease than others.