If the indicator is black then it is worth charging the battery prior to replacing it, then having a proper test carried out at a battery centre where they will measure the CCA (cold cranking amps). Don't take in a discharged battery as it is not possible to test it properly no matter what they tell you.
The indicator is nothing more than a built in hydrometer, that measures the specific gravity of the electrolyte in one cell. Black usually means that the battery requires recharging. (could be knackered, but certainly worth charging first)
65% of battery failures are due to the drive cycle of the vehicle, ie being used for short journeys, often with an electrical load applied. Basically you end up taking more out of the battery than you return with the alternator, and over a period of time the voltage decreases. you need 12.5v or above to avoid sulphation occuring (corroding of the plates). A fully charged battery is 12.66v (any more is considered surface voltage). Bear in mind that only 0.7v separates a fully charged and a fully discharged battery. If you use the vehicle infrequently, or primarily for short journeys, then a decent switch mode (automatic) charger is a worthy investment to keep your battery in tip top condition.
I often change batteries for people (at their insistence) which are ten years old, and after a decent charge, they are performing as well as they were the day they were new.
All this aside, if you want to be absolutely certain, then replacing a battery that is 7 years old makes sense. Don't scrimp, go for a decent brand, Bosch, Varta or similar with 4 or 5 years warranty. There are millions of cheap Chinese batteries about, that purport to be this or that, with fancy labels on them, but invariably when we test them they are crap. Poor quality internal construction using inferior materials, and a sub-standard casing.
As has been suggested, batteries are at the very heart of a modern vehicle, and even a relatively small drop in performance can cause many many problems before you get that dreaded click and no start. Poor running, electrical gremlins, alarm/immobiliser issues and more have all been cured by doing nothing more than fitting a new battery, or properly charging your existing one.