Since, the previous owner hasn't provided the blueprint, who else might have a copy of this document ?
Occasionally local building authorities might; but unless it's relatively new construction in my area, at least, they toss 'em after a decade or so. If your house is part of an organized development, the builder's name might be known. If they're still in business, they might be willing to sell you a copy of the plans, or steer you to the vendor of the plans if they were purchased from an outside source.
If you believe your house was designed by an architect (and you know which one), perhaps you can buy a set of prints.
Except for houses of historic importance, it would be very rare to discover actual plans for any house built before the beginning of the 20th c. There were books with published house plans circulating in the 19th c. and early 20th c. If you know your house's general style it's possible that you may find some examples similar to your building in reproductions of those books.
The blue print may not indicate very much anyway unless the house is very new.
Older plans often relied heavily on the carpenter doing the framing to figure out how to do the job.
Some newer plans have more detail, but not all.
If the place was built using 'per se' rules from the building code there may be very little detail.
The building code specifies joist sizes, spacing, and spans, and the same for studs.
A framed roof can also be built straight out of the code.
Variations are what often produces the need for detailed drawings (and engineering).
I often find that houses were not built according to the original design documents and as-built documents are rarely prepared regardless of the age of the house.
I have a set of the original cyanotype blueprints and written specifications for my house from 1891 but many elements were modified during construction (some noted on the documents) or in the early part of the 20th century.
Complete documents are interesting to have but I always measure the layout and structure and prepare existing condition drawings before starting any design work. No one wants surprises after construction starts.