A blue water boat is a lot of different things to different people. Almost anything can be found sailing offshore and a lot of the boats doing trips are not what is considered offshore capable by the common definition. I think the most important thing is the skipper. A skilled skipper will survive with a marginal or even poor boat while an incompetent skipper will sink even a great boat. So my pat answer to a question like this is to say that if you need to ask the question you are not ready for offshore sailing.
There are a lot of different ways to sail a boat. Your tactics for storm survival for instance depends on a coordinated plan and the type of boats matters very much to the plan. If you think heaving to in extreme weather is the best way to go then you donít want a fin keel boat with a spade rudder because that will almost never heave to during a storm. You will want a full keel or a full keel with a cut away forefoot and skeg hung rudder. On the flip side if you want to run then a full keel might not be your choice because of the quarter wave you get when running fast in heavy weather. Now a fin might be a better choice especially if you think towing a drag is a good idea. Of course the area you are sailing in will now make a big difference. If you are crossing oceans running in a storm is fine but in coastal work you may not have the room to run so a full keel boat which allows some choices for storm tactics might be a good idea. How much comfort and room you want also makes a difference in choosing a boat style. The more comfort and amenities you want means the more room you need and that means a longer light boat or a heaver shorter boat. You buy a boat by the pound but you pay to own her by the foot. Larger light boats cost the same to buy as short heavy boats but you will spend more to run a longer boat. The amount of weight it takes to support a person per day offshore is a fixed number so a boat with higher displacement is affected less by the crew and stores then a light displacement boat until you get into much larger boats.
Read as much as you can and sail on as many boats as you can to get some background. After a while you will start asking more pointed questions and start forming a game plan for both tactics and route planning that will usually narrow it down for you. You will get to a point where you, not everybody else, are comfortable with a style and you will know how she handles under all conditions and then you are ready to pick a boat.
Good luck and all the best,
Study the history of naval architecture and move forward knowing what didnít work before.
Donít waste time making the same old mistakes but instead make new ones and to insure your place in history be sure the mistakes are big ones.
Never design a mast that is weaker then the boat
Never design a boat that is weaker then the mast
Never listen to someone describe why your project will not work unless they can show you the broken pieces of their own version.