Yes, it is really blood!
During the second half of your cycle, the lining of your uterus (endometrium) begins to thicken in preparation for a fertilized egg. By then end of the cycle, if this does not happen, the hormones progesterone and estrogen begin to fall and that thickened endometrium begins to slough off.
What you see in your menstrual flow are the cells from the uterine lining and blood from special arteries in the uterus.
Sometimes it just doesn’t look like the blood when you have a cut. That’s because in addition to losing 4 to 12 teaspoons of blood per cycle, you’re also shedding the lining of your uterus.