Finally, after half a year of enduring this stupid feature, I've finally found the answer to why Blogger automatically makes my uploaded pics brighter. According to a poster on the Blogger Product Forum, Google Plus now integrates the Auto Enhance feature which automatically edits your uploaded pictures (adjusting the brightness, saturation, level, adding filters, etc).
To disable this feature:
1. Connect to your Google+ account.
2. Click on Home > Settings.
3. Scroll to the "Auto Enhance" section.
4. Uncheck "Automatically enhance new photos".
Enhanced images have to be uploaded again, unchecking the box doesn't undo the executed enhancements. Alternatively, you can try uploading your images in the .PNG format.