No the printer is not mixing blacks to offset low ink. In fact, the system used to determine how much ink is left is quite conservative, and typically there is as much as 5-10% of the total ink remaining when the printer first reports it at 1%. With a 350ml cartridge there could be as much as 35ml of ink. The 1% is just an estimate and is not very accurate. As has been mentioned, just print until the printer stops and tells you to change ink. You'll be surprised how many prints you will probably be able to print until this happens.
On the 79/9900, the LK inks will always need to be replaced first for two reasons. First in the initializing set up the printer first charges the MK/LK channel. Then it switches to PK/LK, so the LK ink channel gets charged twice, using a about 30 to 40ml of extra LK ink, which is a large percentage of the 110ml starter cartridges.
Second, the printer consumes more LK ink than any other color - often more than twice as much. LLK is probably next. I recommend the use of the 700ml LK cartridge when using the 350ml cartridges ... it will last about as long as the 350ml of most of the other colors.