If I blend Marinara sauce and then add some cream will that make a decent tomato soup?
Sure it will.
Could we define "decent"? My answer is no. It might improve the sauce but it won't be a real soup. A soup needs to be "looser" than marinara. Of course you could try the prison technique of adding hot water to ketchup. Either way it's not going to be good.
I should've added a "JK" to my answer. Hey, where did you learn the prison technique?
In jail ma'am. Down by law.
As much as appreciate the advice I was given it was incorrect as far as I am concerned. I put the Marinara sauce into the blender, blended it for a while, then added some veg stock and continued to blend, then added to a pan and put in some double cream, it tasted great.