When using an instrument to make a measurement, there are basically two sources of uncertainty: those that come from samples and those that come from the instrument itself. An instrument blank is just a blank that is used to determine the noise level of the measurement that is contributed from the instrument. A method blank is usually the same substance as an instrument blank (i.e., just solvent) but it is carried through the entire sample preparatory and possibly experimental process. You might properly call an instrument blank just a blank, and a method blank a negative control, or some such. The reason this is done is to distinguish between the instrument detection limit (IDL, the basic noise level of the instrument) and the method detection limit (MDL, the effective noise level that takes into account contributions not due to the instrument itself - e.g., laboratory contamination). Knowing both of these values can help the analyst track, and find ways to mitigate, different sources of error in their measurement. E.g., if the MDL and IDL are very close, it tells you that your laboratory procedures are very good and your measurement is as close to "perfect" as you can get it without buying a better instrument. On the other hand, if the MDL is considerably higher than the IDL, then you know there are significant sources of error or contamination due to your procedures, and so your method can be improved without investing in newer equipment.