from PyQt4.QtGui import *
from PyQt4.QtCore import *
import sys

class Bar(QWidget):
    def __init__(self, parent=None):
        QWidget.__init__(self, parent)
        self.vbox = QVBoxLayout()
        self.hbox = QHBoxLayout()
        self.bar = QProgressBar()
        self.bar.setRange(0,100)
        self.bar.setTextVisible(False)
        self.bar.setValue(75)
        self.vbox.addWidget(self.bar)
        self.layout = QVBoxLayout(self)
        self.layout.addLayout(self.vbox)

if __name__ == '__main__':
    app = QApplication(sys.argv)
    bar = Bar()
    bar.show()
    sys.exit(app.exec_())

self.bar.setStyleSheet("""QProgressBar::chunk:horizontal{background: black;}""")