Hi, and thanks for your medical question on eHealth's Ask a Doctor forum.

A patch that looks like dry blood or blood patch at the back of throat is generally due to pharyngitis or inflammation of the pharynx (part of the mouth cavity seen behind the tonsils). This also causes difficulty in swallowing especially if this also involves the upper part of throat.

Warm saline gargles, anti inflammatory drugs and anti-allergics help control the symptoms. Drinking hot soups, herbal tea and warm lemonade also helps.
If symptoms persist you should consult your PCP. Hope this helps. Take care!