The presence of a bleed during your pregnancy
raises concern about a threatened abortion
Which is why your doctor has added primolut injections to your treatment regimen.
The presence of black blood, which could have been thick with the presence of clots is old blood.
This again supports the possibility of a threatened abortion. An irregular uterine bleed is also one of the side effects of using primolut, but this was present before you started off with the medication, so I would still attribute the presentation to a possible threatened abortion.
Please discuss all safety and precautionary measures with your doctor.
Hope this helps.