This is a tricky question for multiple reasons.

First, the idea that plants need sunlight does not really come up before the advent of modern science, or at least not before the 17th century (see below). [Disclaimer: I can only speak about the so-called Western tradition of science.] Before then, Aristotle had dominated the discourse about plants and it was his opinion (and that of many of his followers) that plants would directly absorb all nutrients from the soil, mixed with water. Since Aristotle was the dominant scientific influence until the time of Newton, people were led to believe that sunlight was not necessary at all for plants. 

Second, this is even kind of true when you widen the scope of which plants you are looking at! Moss can grow in places with very, very little light, such as in dark caves or underneath of stones. If you wanted to prove by means of experimentation that moss cannot survive in a properly dark place, you'd have to wait for many years or even decades before the moss that you have isolated from any light source would be considered properly dead, because moss is incredibly resilient can survive for a long time without nutrients and light. Mushrooms can even grow entirely without sunlight. (That mushrooms should be classified in their own kingdom and that they are not plants, strictly speaking, is a rather recent development in biology, an idea that came about in the 20th century.) In fact, sunlight can outright kill mushrooms! People always understood the trivial fact that different species have different needs for nutrients, and the idea that some plants require lots of sunlight while others can seemingly go on without any observable amount of sunlight would not be strange or questionable at all. So an early scientifically minded observer would note that they had, for example, a dark basement wherein no crops and flowers could grow, but moss and mushrooms could grow there very well, but this was not requiring any explanation. - Also, there are plenty of plants that grow under water and it is only with 20th century diving technology that we know that plants cannot grow at any depth, so earlier people had reason to assume that the dark depths of ocean floor would be covered with plants. Case in point: Jules Verne writes in his book "20.000 leagues under the sea" (1870!) that there are wild forests on the dark ocean floor and he emphasizes the darkness there. Link.

Third, people had some ideas about the nature of sunlight that sound a bit weird today. At least in the 17th and 18th century, they were usually conflating light, heat, and the ether, which was conceived as some kind of very subtle fluid that could permeate all kinds of matter. From the discovery of the vacuum and the quarrel between Robert Boyle and Joseph Wright in the 1660s on it was a matter of intense debate whether there could possibly be places that were absolutely empty and wherein no ether (and hence light) could be found – a debate that continued throughout Huygens, Newton, Euler, Du Chatelet, Kant, and further. Hence, it was unclear whether there could be a place without any light, as this would also be a place without any heat – or, to put this in modern terms, whether there could be a place without any kind of radiation at all. 

However, despite all these reservations, plants were indeed difficult to explain for many early scientists. People started questioning Aristotle more and more from the middle 1600s on and the general question of the ‘building blocks of life’ started to take center stage in emerging biology. Scientists like Van Helmont, Haller, Priestley, G.E.Stahl, Lavoisier and others were discussing what it is, biologically and chemically speaking, that distinguishes living beings from dead matter, and they were looking at plants as well. The basic consensus was, speaking very broadly, that there was some kind of fire or heat that was burning within a living body and distributed by means of blood and other fluids, but the details were often really wild, for example when they included the phlogiston-theory, which combined principles of acidity and chemical basicity with theories of heat and combustion into one rather occult substance that was used to explain everything that was difficult to explain before the advent of modern chemistry and cellular theory. Herein, external heat (such as sunlight) was seen as a possible catalysator of internal combustion processes, but the details remained shady - Immanuel Kant famously proclaimed that there could never be a Newton of a blade of grass, which means that the explanatory precision and systematicity of physics was simply deemed impossible for biology. A noteworthy precursor of the modern theory of photosynthesis was then proposed by Jan (or John) Ingenhous in the late 1700s, who also was the first to notice that plants require oxygen to live (not air, as there are plants that grow under water).

In short, this is a very interesting topic! While this fact seems so natural and intuitive to us, it actually requires a rather complex scientific background before it can be seen as worthy of discussion and explanation in the first place.