Sheikh `Abd al-`Azîz b. Bâz
If this cycle that you describe takes place in under fifteen days and is consistent every month, then it should all be considered menstrual blood.
If your blood stops some days of the month and appears in most other days, the trace of purity would be the total stoppage of blood even if you do not see the white sign of purity. Your menstrual period starts with the flow of blood. If the blood is running continuously then this is called istihâdah.
If you had a regular menstrual period before suffering from continual bleeding, then you have to count the days of your normal period.
If you did not have a regular period and the blood varies from black to red, you have to count as your period the time that you see the black, thick blood which has a distinct smell.
The Prophet (peace be upon him) said: “The menstrual blood is black and easy to identify.” [Abu Dâwûd, al-Nasâ’î and others, the original hadîth is related by Muslim (333)]
In case you do not have a menstrual period at all and the blood comes out in one state, usually red or yellow and red, then this is istihâdah. You should abide by the rulings set for women suffering from istihâdah.
The woman in istihâdah should wear something on her private parts to prevent blood from contaminating her clothing and then she should engage in all her normal acts of worship such as praying and fasting. She may have sexual intercourse with her husband.
And Allah knows best.
Source: Islam Today