Linear Blend: If you select a block of colors in the color palette (several adjacent rows or columns), then click the Blend button, you'll get a Linear Blend. The colors in the upper-left and lower-right corners are unchanged, while the colors in between are blended between the two colors. The result is that the colors selected in each row are the same color.
Four-Corner Blend - An alternative method of blending is the four-corner blend. This is done by holding down the Option key while clicking the Blend button. In this method, the four colors in the corners of the selected colors are unchanged, while the colors in between are blended from the corners. The result is that every color will be different.