We visited just in time at the peak cherry blossom (April 20 - 25). By the way, this is an early for Morioka cherry blossom according to a local. It was beautiful, especially you walk through cherry blossom tunnels. I recommend go both day and night as during cherry blossom festival, the cherry blossom trees are lit up, and you can observe unique Japanese tradition (Hanami, people gather with foods and alcohol in cheery atmosphere). There are food vendors, but the price is a bit more, so we bought alcohol/food from convenience stores nearby.
I also recommend to stop by weekend. We happened to see Tsugaru Shamisen performance on Sunday during Earth Day or Green Festival. It seems this site is often used for public festivals.
The site has no castle as the name ("ruin") suggests. Now only stone walls remain. It's a nice walk through multi-layer structure and Koi fish pond near the Sakurayama Shrine.