There is significant folk lore around the wonderful benefits and health powers of garlic and onions. I find garlic is fairly strong tasting, and some may find it hard to eat any significant quantities. Clinical type trials indicate some sort of effect, but usually at fairly high dosages, perhaps more than most folks would want to eat.
My personal experience is that a diet of garlic and onions in soups I have daily seems to have a blood pressure lowering effect. Don’t expect to see large amounts of blood pressure reduction, but it is probably good for a few blood pressure points of reduction.
Garlic apparently can thin the blood similar to aspirin, so one does have to be careful with blood thinning drugs (warfarin, aspirin), etc, ask your doctor. It is also often recommended that garlic intake be reduced a week before and until week after surgery.
http://altmedicine.about.com/cs/herbsvitaminsek/a/Hypertension.htm
http://www.medicine.ox.ac.uk/bandolier/booth/alternat/AT025.html
http://chinesemedicinenews.com/?p=811
http://www.ncbi.nlm.nih.gov/pubmed/8361870
http://www.biomedcentral.com/1471-2261/8/13