Explanation of the ‘cherry red spot' in Tay Sachs disease
Short answer  


The fovea centralis is colored red in both the healthy retina and Tay-Sachs affected retinae. The macular region around the fovea has a yellowish hue. You are confusing fovea and macula in your question. 
The reddish hue from the fovea comes from the well-vascularized choroid layer below. The choroid is not black, it is colored red due to the presence of many blood vessels. You are confusing choroid with the retinal pigment epithelium, which is indeed black.  


Background
The macula includes the fovea at the center, also called the fovea centralis, which forms the very center of the retina. The macular region around the fovea includes the perifoveal and parafoveal areas. The macular region around the fovea has a slight yellowish appearance to it due to yellowish pigments present in the cone axons (Fig. 1) (Kolb, 2012).


Fig. 1. Healthy retina. The macular region is seen in the center, The parafoveal region has a slight yellow/orange hue to it. Note that the landmark yellow circle at 4 o'clock, with the vessels sprouting from it, is the optic nerve head. source: Webvision.

Below a Tay-Sachs affected retina showing the hallmark cherry red spot (Fig. 2):


Fig. 2. Tay-Sachs affected eye. The fovea centralis appears red. source: Prezi  

The only normal part is the fovea centralis, appearing in its native red color, i.e. the cherry red spot. 

The choroid plexus is well-perfused and is, therefore, red. You are confusing it with the pigment epithelium, which is pigmented (Fig. 3).


Fig. 3. Layers of the eye. RPE = Retinal Pigment Epithelium. source: Retina Eye Specialists

Reference
- Kolb (2012), Simple anatomy of the retina. In: Kolb et al. (eds.), Webvision. The organization of the retina and visual system. Salt Lake City, UT, Moran Eye Center