Postby Fuyu » Tue Jul 08, 2014 10:40 pm
Transparency is a pretty complicated part of background modification.
Like Blank said, you need to use a grey color for the mask instead of black. Let me explain to you how it works. Usually, when you're making a mask for a block/background, the black color is used to define which pixel of the actual graphic should be printed normally, and white is used for those that shouldn't. Choosing the middle one, which ends up being grey, you should get a great transparent color.
But, like I said it's a pretty complicated addition, way more than anything related to SMBX GFX modification. Overall because there are various mixtures of grey, or simple grey variations that end up showing multiple but different results. I wouldn't suggest using this feature unless you learn more about masks, but I think it should be ok if you still want to use transparency.