If a texture is 'incomplete', only black gets sampled. One common reason is that mip-mapping is activated for filtering but the texture has no mip-maps. For most texture types using mip-maping as a filter mode is the default! So either call generateMipmaps() on them or change the minification filter.
Why are my textures upside down?
The coordinate system of OpenGL starts at the bottom on the left while the coordinate systems of some fileformats start at the top left corner. A coordinatesystem transformation is not performed while loading the image. If the image gets loaded manually using an intermediate TextureData object, TextureData::flipVertically() can be called.