More or less.

If you want total control over the way your book will print you have to download Blurb's icc-profile, install it, and use the softproofing feature in Photoshop.

Lr does not allow for softproofing to Blurb's icc profile since this profile is a CMYK profile. But in Photoshop CSxx you can.

sRGB is only an approximation of what your printed book will look like. Be particularly careful with shadows. When you calibrate your monitor the luminance (brightness, intensity) of your monitor should not be above 120 cd/m2 - better at about 110. If your monitor is set to a brighter value all your prints will come out too dark.

After adjusting the brightness of your monitor evaluate your photos for the book carefully to the shadows so that they don't print too dark.

If you have a good printer available at your place do some test prints: Set Color Management in the print Module of Lr to <Printer Managed> and set your printer to sRGB. This will give you a good idea what your book will look like. Naturally, it also depends on the paper that you choose for your book - and you can't simulate that with you rprinter.

The OOG gamut overlay is not accurate nor necessary! In terms of accuracy, save out say a document in sRGB, open in LR then soft proof for sRGB and turn on the OOG overlay. See anything? You shouldn’t. It is a bug, one you’ll see in both LR and Photoshop. For that reason alone, the OOG overlay should be dismissed although there are other reasons to skip the use.

Trying to remove the OOG (assuming they were correct) is kind of a waste of time. The ICC profile that will convert the data will do this faster, with far more precision and with options (Perceptual or Relative Colorimetric). The idea that you need to remove them is based on very, very old school image processing circa 1997 and earlier before Photoshop 5 had soft proofing and the ability to use profiles to convert our data. As an example see: http://digitaldog.net/files/LR4_softproof2.mov

In terms of Blurb output, well any kind of soft proofing, let alone dealing with OGG is a big waste of time. First, you have to upload sRGB to them, they will convert that data to CMYK using who know’s what ICC profile for what print conditions. Now they do provide a single CMYK profile for soft proofing that you can’t use in LR (doesn’t support that color model) but you could do this in Photoshop. But why? There is no way this single profile defines all the possible output conditions they provide (every paper requires it’s own ICC profile). So basically you just upload the data you see in LR, hope for the best. Might as well let LR convert to sRGB upon upload, it will use a RelCol rendering intent because that is the only option.

The output I got from a Blurb book with some pretty tough reference images and synthetic images I use to test such output was pretty disappointing but this had nothing to do with OOG. The output just wasn’t good! The difference in color and tone between the same image on the cover and printed inside the book was shocking. Maybe I just hit them on a bad day, but I wasn’t at all impressed with the color, and the same book images set out in Aperture was vastly superior!

OP: So the OOG display is conservative, warning for things that are slightly in gamut.  If soft-proofing shows no warnings then you’re ok.  If soft-proofing shows large areas with problems then you’re probably not ok.  If there are a few areas then you may or may not have problems that are noticeable.  For an image showing large areas out of gamut with respect to sRGB, exporting as sRGB, re-importing and doing a side-by-side in Library will show what happens to those colors during the profile conversion to sRGB.  It is unlikely the colors stayed the same, and it may or may not be better than what you could so by hand.

AR:  Did you send your Aperture output to Blurb after using their CMYK profile or how are you comparing Aperture to Lightroom?