First of all a flat pattern indicates a 3-dimensional shape, probably sheet metal. The shapes you listed, a circle and a rectangle are 2-dimensional.
Second your question is not worded well and is somewhat confusing a blend and a hollow do not seem to refer to the same thing - at least not to me.
Third you are obviously asking a software question and we have no idea what platform you are designing on.
Fourth you are in the completely wrong group - go the the Engineering Computer Programs forums, find the right software package that you are drawing with and post a well written question there.
