You know, I'm not sure our question was answered properly....
As simply as I can say it, after being at sea for 25 years as a Master Class 1, a blue water yacht is one that is designed and built to protect you from the elements.
Stability, Strength and Security are the primary concerns here. They all dilute down to heeling angles, weight, sail area, watertight divisions and reliability and /or redundancy of your systems. The most important word in safe passage, is "safe".
The gents who set off to Hawaii in their boat were, in retrospect, unprepared despite a year spent in planning. The time in planning doesn't mean anything if taxpayers have to pay to have you rescued! You may as well set out in a bathtub. The good news is they survived to write about (and learn from )their ordeal.
In my view, a blue water boat is one that meets the CE criteria, its one that Class Societies (Lloyds, DNV, etc) are prepared to certify for ocean passages.
Many factors contribute to making it "safe". All the ABYC wiring codes, Life Saving and Fire Fighting Appliances (LSFA) bilge pumping systems, fire systems, redunacy of systems, availabiliy of spares to name but a few.
I recently bought a 21 year old fibreglass pilothouse boat that is so well built, so well maintained and so heavy, solid under foot and "substantial" - A name we gave to our fat cat, - that I'm prepared to go anywhere in her, - other than ice regions.
Long range on engine alone, (2,000 miles) - Heaps of fresh water and huge freezer. Soild, reduced rig
, - not as fast as Steve and Linda's Beuwolf, but comfortable and manageable by a single person - (should one be incapacated)
This is a blue water cruiser.
To misquote Neville, "But, that's just my opinion"
Buy well, be safe.