Rather than trying to predict the exact date the blossoms will be out in Ueno, it may be easier just to plan a trip in early April and then wait until the moment you arrive to find out where the best blossoms are at that exact moment. There's a chance that the trees in Tokyo will be in bloom - but even if they're not, you'll be able to travel to some place within a short train ride to see a fine sakura display. (Actually, it's much nicer to see the sakura rather further from the madding crowds than Ueno park).