What is the Black Drop Effect?
When Venus is near the inner edge of the Sun's disk, its edge appears to smear and make a hazy connection with the solar disk. This makes Venus look like a black drop dripping off the edge of the Sun. This smearing prevented astronomers from timing accurately the moment when Venus made "contact" with the edge of the Sun, dooming any chance of obtaining an accurate measure of the Earth-Sun distance.
The effect is caused by a combination of factors, including Earth's atmosphere, the diffraction of light waves within a telescope, and the limitations of the human eye.
To see this phenomenon, hold your thumb and forefinger up to a light and slowly bring them together. Notice how the gap between them seems to smear before they actually make contact.