Homemade/DIY Frozen Vegetables: Can steaming be used instead of blanching?
The part about blanching that is important is that the vegetables are briefly cooked and then immediately doused in cold water to stop the cooking process. Processors use boiling water as it is easier to manage and you can put flavorings and other additives (preservatives, color enhancers, etc) to the water to get the effect desired. There's no reason you can't steam them instead as long as you dip them in ice water to arrest the cooking process. There's some things that steam blanching wouldn't be good for, like removing the skins from tomatoes, for that boiling water is the way to go, but for preparing vegetables for the freezer it will work fine. 

Some considerations:


Industrialization of the process: it's easy to set up a line with a pot of boiling water and a bowl of ice water while using a basket to immerse. Steaming won't be as straightforward, and will be more time consuming. If you have a lot of vegetables you may want to consider boiling instead
Safety: You will need to be able to move the vegetables from your steamer to the ice water quickly without getting scalded, so think about your setup and tools