Walaikum assalam wa rahmatullah
The fiqh of hayd (menstruation) varies greatly between the madhabs.
According to Hanafi madhab (the only ruling which I know), valid blood lasts for 10 days (240 hrs to be exact). Everything outside this is regarded as istihada - ie not mensus - a woman is obliged to perform her salat (and fasting in ramadan) during this time.
I dont know which madhab you follow, so please, if you are not hanafi, do not regard this post of any significance.
According to Hanafi fiqh, a woman has a habit (adah) which amounts to her last valid blood and last valid tuhr. If you are seeing anything other than white beyond 10 days of blood, then return to your habit.
eg, if your habit is of 7 days if blood and 21 days of tuhr, then all blood that exeeds 10 days will be regarded. as istehada. and the like. Ofcourse the knowledgee of this will be gained in hindsight, ie when you bleed for more than 10 days.
If the black color becomes clear in 240 hrs of the beginning of first show of blood, then there is no need to worry. Just wait till it gets clear, then perform your ghusl. So in any case wait till 240 hrs, then decide what to do.
Hope this helps.
Wanu nazzilu minal Qurani ma huwa
Shafaa un wa rahmatun lil mo'mineena
wa la yaziduzzalimeena illa khasara.