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Cutting away the branches and stems that have knots is only the first step. The second step is to apply a fungicide to the tree, which will require a discussion with your local cooperative extension who can tell you which fungicide will work best for your region.

For more information on black knot, please visit the following link:
https://www.gardeningknowhow.com/edible/fruits/plum/black-knot-tree-disease.htm

Yes, the okame cherry can be kept smaller in a pot or planter. You can even make it into a bonsai. Some strategic pruning is all it takes!

Here are some great articles on how to grow trees in containers:

https://www.gardeningknowhow.com/special/containers/how-to-grow-container-trees.htm

https://www.gardeningknowhow.com/ornamental/trees/tgen/growing-trees-in-containers.htm

https://www.gardeningknowhow.com/houseplants/bonsai/bonsai-basics-starting-your-own-bonsai.htm

What kind of cherry tree do you have and is it the same variety as those in the neighborhood around you?

What has the weather been like? Cold weather can affect the onset of cherry blooms and this is most likely what is happening here, so I think a "wait and see" approach is best for the time being.

The short answer is no - the blossom will soon be over and then the tree can be pruned quite safely, but not too hard!
Henry Villiers

I am wondering if the brown rot fungus is afflicting your trees - I have included a link to an article about brown rot below so you can make a determination. Brown rot looks like this:

https://extension.psu.edu/plants/tree-fruit/news/2012/brown-rot-preharvest-alert/image_galleryzoom

Preventative measures, such as applying a fungicide in the early spring prior to onset of flower buds appear is key.

https://www.gardeningknowhow.com/edible/fruits/peach/brown-rot-fungus.htm