The PixelBlock object is designed to handle generic pixel arrays from any raster data source. This means it must be able to handle single and multiband data, as well as support different pixel types. To support multiple bands, or planes, of raster data, the PixelBlock provides a separate array for each band (plane) in the raster. You can get the pixel data from the pixel block, modify the pixel values, and write the pixel block with the modified pixel values to a raster band. If the pixel block is created from a Raster, the modified pixel block can be read using the Raster.Read method and written to the raster dataset using the Raster.Write method. For a small raster dataset, the size of the pixel block can be the size of the entire dataset, which can usually be held in memory at one time. When working with large raster data, the best practice is to divide the raster into small pixel blocks and read and write these block by block using the RasterCursor Object.

Reference

PixelBlock Members
ArcGIS.Core.Data.Raster Namespace