Can ventricular tachycardia cause blackout?

Yes. Black-outs (or syncope) are often caused by transient reductions in blood flow to the brain from a variety of causes. With ventricular tachycardia, the abnormal rhythm may be associated with a reduction in cardiac output and a temporary reduction in brain blood flow that can lead to syncope.
Yes. Yes, it certainly can. Vt, when fast, gives the heart insufficient time to fill. Consequently, cardiac output (the blood pumped per minute) falls and the brain can be underperfused resulting in blackout. Moreover, VT can be an unstable rhythm that deteriorates to ventricular fibrillation resulting in death. If you have vt, you need to be under the care of a cardiologist or an electrophysiologist.