If the aortic semilunar valve is blocked, where would blood accumulate

Blood would potentially accumulate in the left ventricle, if the aortic semilunar valve were blocked. This is because the normal flow of blood in the systemic circuit, in which contraction of the left ventricular cardiac muscle tissue on a filled chamber, leads to quick opening of the aortic valve and blood escaping out into the aorta and subsequently into all systemic arteries (because of the high pressure gradient).
If blood cannot escape adequately through this valve and out into the aorta, then the left ventricular pressure would be excessive, and blood would eventually backflow into the left atrium by forcing the mitral (bicuspid AV) valve to open upwards (prolapse or regurgitation).
This is one of the pathophysiologic methods, medically seen in aortic stenosis (AS), that contribute to left heart failure (LHF). Stenosis refers to a hardening and narrowing if the valve over time.
This progression only worsens the situation, as the weaker-walled left atrium gets congested with the backflow of blood, it sends the extra pressure back into the pulmonary circuit, leading to pulmonary hypertension. Symptoms such as shortness if breath (dyspnea) and cough with frothy sputum can be symptoms of left-sided heart failure (LHF).