A blank titration is done without the analyte present to check for possible sources of error in the "blank" solution. For example, deionized water is slightly acidic and may affect the results of an acid base titration (if you want a highly accurate concentration). So you would do a blank titration to find the concentration of H3O+ in the water and use it to correct the conentration of your analyte.
A back titration is used when it is diffucult to find an endpoint in a normal titration (for example, if the analyte is not very soluble in water). You react the analyte with an excess of titrant and then titrtate the excess titrant. Subtractinf the moles of excess titrant from the original amount you used gives the moles of titrant reacted with the analyte. From that you can figure out moles of analyte.