Blood that that is emitted out of a woman's vagina during menstration. (Period Blood)
Bladge is not normal blood, because when it comes out of a woman's chatch along with old eggs and whatnot, it becomes something that is SO much more disgusting. Sometimes it even has a much darker color than normal blood, depending on if the girl has weird uterus problems or not.
Bladge can also be used as an alternative lubricant for sex. Just make sure you turn off all the lights and wear a condom that you'll promptly throw away after you're done. Failure to follow both of these guidelines will result in lots of vomiting, and perhaps even erectile dysfunction, because bladge is really gross. If you see it or smell it, you may get sick.
Also, it is a good idea to do the dirty deed on old furniture that you don't mind throwing away the next day. Make the girl throw it out too, 'cause she's used to seeing that disgusting crap every month, and is therefore more immune to the horridness of it all.
Barlow: "Yea, Meagan's bladge is all over it!
Stooge: "Eww! Why did you have sex during her period?!"
Barlow: "She didn't tell me until after we were done!"
Mattei: "I wish I knew it was there, cause I had sex on your bed the next day. Now my ass is covered in day-old bladge!"