When I turn on shaders all the items are now black blocks.

Same when the gamma is on 100% mode

This is an issue with optifine and not with this mod.
This mod doesn't change anything that could cause an issue like that.
When the gamma is set to 100% it'll be exactly the same as having your brightness setting on 'bright' without this mod.
It seems like the latest optfine fixes some issues related to shaders, so make sure to update it.
Also keep in mind this mod won't work with shaders anyway.