Both look great...it's entirely up to you. Part of it is going to be how much blue do you want to see and how monolithic will the fireplace be in the room. In your case, I'm leaning toward blue bookcases only.

I think one deciding factor would be the design of the fireplace. The big thing I learned was about having space for the mantel to die into. If you look at the cream fireplace, you'll notice it is against drywall that is painted to match, that is proud of the bookcases, and is wide enough to accommodate the mantel molding. That allows the bookcases to be more recessed and a separate color as they are each a separate unit.

In the blue example, it appears that the frame that the mantel dies into is also the frame for the bookcase, uniting them as one, thus the single color works.

Also note that in your sketch, you do not have an overmantel whereas both inspiration pics do. So the wall above can be painted to match or not.

If it's any help, here's what we did in our library.

We went with cherry and had the fireplace proud of the bookcases along with the sheet rock overmantel. The flanges of the bookcase were widened to accommodate the mantel dying into it. We painted the sheet rock the same as the rest of the walls.

Your rooms are well laid out and very thoughtful. I think you're going to really enjoy being there.

Thanks Annie,
to address some of the points you brought up, the fireplace 3rd of that wall is 6', and there will be room to die the mantel into the drywall without extending to the built-ins. Also, the framing is done and the FP section is approx 4" forward of the elevation for the front of the bookcases, because I'd asked to have the bookcase recessed.
There will be an overmantel or whatever to call it. Our builder has this picture and it'll be trimmed out very similarly. The problem is I don't have the skill nor the software to mock it all up correctly, and all I had to work with was a drawing that I photographed from our cabinet maker on the actual cabinets, not the trim out. And I hate writing a book with too many pictures for people to have to wade through on the initial post.

[Traditional Living Room[(https://www.houzz.com/photos/traditional-living-room-ideas-phbr1-bp~t_718~s_2107) by Rockford Interior Designers & Decorators KannCept Design, Inc.

The actual ceiling is vaulted in this room, so we talked with the builder about building this with the trim so that the eye has a place to stop, and that triangle of wall will hopefully recede off into oblivion. I'm not generally a fan of vaulted ceilings, but we needed a dormer to bring light down into the room as the entire house except the bkfast bay is wrapped in porch. The interior would be intolerably dark without the dormer.

BTW, thank you for posting pics of your library. It's beautifully done and well thought out. And I love that your shelves are actually full of books! The trend seems to be to fill shelving with display items. I absolutely love your American cherry wood.