What type of colorful trout bluish in sea water at which point it is called steel-head?
Yes, it is. One member of the trout family, the rainbow trout, will occasionally go out to sea, and return in two or three years to spawn - it's appearance has changed by this point to where it is called a Steelhead trout. But it still begins and ends it's life in fresh water. Most other trout are strictly freshwater fish.
When you have a single piece on any point it is vulnerable. It is called a "blot". If you put a second piece with it is is called "making the point". Same thing if you move two pieces to the same empty point. If you have two or more pieces -- a "stack" -- on a point, it is called "owning the point".