When Gregor Mendel was performing his first Genetics experiments on pea plants, the idea of incomplete dominance was not known. Pea plants show two distinctive traits, and only two traits, for each characteristic that Mendel studied in his garden. For example, the flowers of a pea plant are either purple or white and never a blend of the two colors like lavender. Since the traits did not mix, Mendel's work just showed complete dominance or complete recessive traits. There was no intermediate phenotype for the heterozygous plants.
Even though Mendel did not try to explain traits that did blend, other scientists eventually did. The discovery of incomplete dominance is usually credited to German Botanist Carl Correns who studied four o'clock plants. Instead of having only two colors of petals on the flowers, there was an intermediate color that would show up that corresponded to Mendel's genotype ratio of 1:2:1 instead of his pheontype ratio of 3:1. This showed that each genotype corresponded to its own unique phenotype and that the heterozygotes were showing a blend of both alleles instead of one being completely dominant over the other.
Incomplete dominance was actually seen and recorded long before Carl Currens published his works and even before Gregor Mendel worked with his pea plants and published his findings. Since there was no discipline known as Genetics at that time, however, it was not fully explored. Scientists dating back to ancient times discussed the blending of traits in their writings which could be attributed to incomplete dominance. However, it wasn't until after Mendel and Currens that the term "incomplete dominance" came into common usage and the mechanism for how it worked was known.
Most examples of incomplete dominance involve color in one way or another. Flowers are most often cited as examples of incomplete dominance. For instance, snapdragon flowers can be either red, white, or pink. If they followed Mendel's inheritance patterns of simple dominance, the flowers would either be red or white in a 3:1 ratio. However, with snapdragon flowers, the ratio of red to pink to white is 1:2:1. This was Mendel's genotype ratio of homozygous dominant to heterozygous to homozygous recessive. This means that the heterozygous plant was actually blending the "dominant" and "recessive" alleles instead of the dominant trait completely masking the recessive trait in the phenotype of the plant. If red and white are blended, the color pink results, just as it does in the snapdragon flower color.