the causes for your blackouts could be,
as energy consumption will increase during pregnancy
to met the needs of mother and baby if food intake gets delayed hypoglycemia
Also when you lie supine position because of pressure of the uterus
on the underlying blood vessel aorta the blood supply to heart decreases leading to hypotension
So, you have to take food at timely intervals and sufficient to your needs.
prefer to sleep in left lateral position which will decrease hypotension.