The problem with the terms heliotopism and phototropism is that the sun or light is the stimulus. Heliotropism is following the sun, and phototropism is moving toward light. This movement is apparently triggered by the absence of light, i.e. darkness is the stimulus.

Is it a gravity response? Negative geotropism? If a plant is completely shaded, growing straight up would be its best chance for finding light...

Hmmm... maybe phototropism has a greater affect on leaves so that without light (or adequate light), gravity is the stimulus that flattens them out or point them downward. That well could be, but I'll bet there's a specific term for it (like etiolation, but of course that's not it). I'd really like to find that term.

Tropism is defined as: a movement by a fixed part of the fixed plant towards or away from a stimulus such as gravity, heat or light, the direction of movement being determined by the direction from which the stimulus originates. A tropism is caused by a positive or negative growth response on the side of the organ nearest to the stimulus relative to the side furthest from the stimulus, and the whole organ then bends away or towards the stimulus respectively.

There are 'few' tropisms, depending on stimulus: Phototropism; Geotropism; Chemotropism; Galvanotropism; Hydrotropism; Rheotropism; Thermotropism; Traumatropism; Thigmotropism. (Hope I got them all?)

Skototropism is growth away from light but I don't think it applies here

I don't think you are going to find a term for this exactly.