Hi Maria,
In 2018, Sakura were still in full bloom by the 1st April. That day I went to Banpaku Koen (north of Osaka) and the sight was still splendid. However, it didn’t last much longer (a couple or three days more). In the end, it will come down to the year (whether it’s a cold or a hot year). In Osaka, the sakura usually bloom between late mid-late March, beginning April.
As for the weather, usually in April it’s getting good. Mild temperatures ranging between 15-22ºC during the day and 8-14ºC at night. In general, it’s pretty sunny, though there are ocasional rains too, so you’ve got to be ready for all-around weather, though not too cold neither too hot.
I wish you good luck with the sakura! If you weren’t lucky and don’t miss the full bloom by much, if you have the chance, you can always go to Kyoto, where it blooms a bit later.
Anyway, I hope you have a great trip!