I wrote this up after trying to help Carnzoid on his post Q: Configuring blood 1 for OSX in dosbox. Hopefully it will be of some use to others until GoG releases an official OSX installer.

Blood for OS X

Note that files and version numbers were current as of 2016.06.09
I did this on a fresh install of OS X El Capitan (10.11.4)
You might have to set your OS X security settings to allow executing Apps downloaded from "Anywhere". Press Command+Space-bar, to open Spotlight and type in Security & Privacy to get in there and make changes you need.

First, Gather the files you will need:

Blood from GoG.com
In your library, in your Blood download options, notice the OS selection. Select Linux and download. You should get a file gog_blood_one_unit_whole_blood_2.0.0.6.sh, I will refer to this later as gog_blood.sh

Boxer (boxerapp.com)
Boxer-1.4.0.zip

Install Boxer. Unzip the Boxer-1.4.0.zip by double clicking on it. Drag & dropp Boxer.app into your Applications folder

Now you will need to extract your Blood files from gog_blood.sh. This is a self-extracting shell executable build for Linux. Unfortunately, it does not work on macOS. To extract the files, you will want to open your Terminal, press command+Space-bar to bring up Spotlight and type: Terminal.
Once you are at the command prompt, change directory to where ever you downloaded gog_blood.sh; probably your Downloads folder. Extract your Blood files to a new directory by typing in the command: After it has finished extracting your files, you can close the Terminal window. The actual Blood game data can be found under “gog_blood_files/data/noarch/data” dive down into noarch and rename the data folder to Blood. Keep this window open so you can drag your Blood folder into Boxer, later.

Now open Boxer. You will be presented with the Boxer welcome screen. It has three icons: Select “Import a new game”
Your window should change and present you with the option “Drop your game here.” Drag your Blood folder into the dashed box area of the Boxer window. Boxer will see Blood’s setup.exe, and thinks this is the installer. Ignore this by clicking the “Skip installer” button. Boxer will now copy your Blood folder into ~/DOS Games/Blood. If you want to give it GoG’s Blood icon, this can be found here: “gog_blood_files/data/noarch/support/icon.png”. Drag and drop the PNG onto the blank CD cover.

Boxer will present you with a window saying: “Congratulations! Your new gamebox is ready to play”… not quite, but go ahead and launch so it can generate it’s DOSBox Preferences.conf file. You should see a DOS prompt, close this window. Go to your game shelf with the Blood icon on it. Right-click Blood and click “Show Package Contents”

You should see a C.harddisk folder and a few files. Be sure that Boxer isn’t running in the background first, close it out of the Dock below. Now, open the file “DOSBox Preferences.conf” with TextEdit. This is where we can tell Boxer the way we want to launch Blood.

Under the autoexec section, and below the second set of commented out lines (they start with #) add in the following lines: When you save the file, you may be presented with a notice that another application is using the file, tell it to save anyway.

Launch Boxer again and brows your games. You should be able to launch Blood from your game shelf.

Go paint the town RED!

Just an addendum, I did try running Blood with the official OSX DOSBox 0.74 release, but I seemed to be getting better frame rates with the Boxer version. Not sure what version of DOSBox Boxer is built on.
Also, editing .bash_profile in OSX was really inconsistent. Thanks Apple!
Addendum 2016.06.10:
Gydion indicated below, that we can use the terminal command, unzip, to extract the gog_blood.sh file. I was not aware of this and had previously suggested uses use the Keka extractor

Believe this should work instead to extract the installer: Does the nightly SVN build fair any better?

Sweet! I will edit the guide above to use that instead. I don't have much experience with Macs, so I was unaware of unzip. When I tried using gunzip to extract the .sh, it threw back an error saying that it wasn't a valid gzip file. 7zip was able to extract it in Windows, so I went looking for an OSX version of that and found Keka. Thanks so much!I did try the latest (now a little old) YKHWong Daum build, but it threw back a missing library error. I haven't tried any of the other OSX builds.

Update: I finally noticed the link to the DOSBox SVN snapshot for OS X Dominus had in his signature. I downloaded that and tested it with the config files GoG included in in the gog_blood.sh archive. I'm sad to report it had the same performance the stable 0.74 build has. i.e. lower frame rates and really choppy video playback with the cut-scenes. I'm sure it's just a configuration issue. But I think I will leave that level of tinkering to the more advanced user and have the above guide for those who just want to get it working.