who came up with this theory?
Charles Darwin.
Blending inheritance
Charles Darwin developed his theory of evolution by natural selection on the basis of an understanding of uniform processes in geology, acting over very long periods of time on inheritable variation within populations. One of those processes was competition for resources, as Thomas Malthus had indicated, leading to a struggle to survive and to reproduce. Since some individuals would by chance have traits that allowed them to leave more offspring, those traits would tend to increase in the population. Darwin assembled many lines of evidence to show that variation occurred and that artificial selection by animal and plant breeding had caused change. All of this demanded a reliable mechanism of inheritance.

Pangenesis was Darwin's attempt 
why is incomplete dominance not evidence for the blending theory of inheritance UNANSWERABLE what is pangenesis theory? According to this theory  each part of the parent's body emitted tiny particles called gemmules, which migrated through the body to contribute to that parent's gametes, their eggs or sperms.
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Blending inheritance [SEP] Darwin's pangenesis
All of this demanded a reliable mechanism of inheritance. Pangenesis was Darwin's attempt to provide such a mechanism of inheritance. The idea was that each part of the parent's body emitted tiny particles called gemmules, which migrated through the body to contribute to that parent's gametes, their eggs or sperms. The theory had an intuitive appeal, as characteristics of all parts of the body, such as shape of nose, width of shoulders and length of legs are inherited from both the father and the mother. However, it had some serious weaknesses. Firstly, many characteristics can change during an individual's lifetime, and are affected by the environment: blacksmiths can develop strong arm muscles during their work, so the gemmules from these muscles ought to carry this acquired characteristic.