The blood inside them is dark red and it doesn't reflect light very well. The blood you see when you get hurt is usually venous blood.
Arterial blood comes out in spurts.
It spurts every time the heart beats. I hope you never see that. It will then become a very poor pump. If you want to know the name for a big flabby heart it is - dilated cardiomyopathy. Sometimes the heart muscle gets thicker but the heart doesn't get much bigger, this is known as hypertrophic cardiomyopathy.