These are two different wounds with two different causes.
The cause is the clearest difference. Venous ulcers are caused by high pressure in the veins of the lower leg. The one-way valves that keep blood moving toward the heart become weak, or the veins become scarred and blocked, so blood flows backward and pools in the legs. Ischemic ulcers, also called arterial ulcers, are most commonly caused by narrowed arteries. This prevents a healthy supply of blood from flowing to the legs, so the tissues do not get enough nutrients and oxygen.
One is blood that can’t leave. The other is blood that can’t arrive.
They look different. A venous ulcer is a shallow sore with a red base, sometimes covered by yellow tissue, with unevenly shaped borders. The skin around it may be shiny, tight, warm or hot, and discolored. An ischemic ulcer is dark red, yellow, gray or black, with raised edges so that it looks punched out. It is deep enough that tendons may show through and usually doesn’t bleed, and the skin on the leg appears shiny, tight, dry, and hairless.
They show up in different places. Most venous ulcers occur on the leg, above the ankle. Most ischemic ulcers occur on the feet and legs, and may appear on legs, ankles, toes, and between toes.
The difference that matters most is in the treatment, and it runs in opposite directions. With a venous ulcer, you will wear a compression stocking or bandages over the dressing. Wear them every day as instructed, because they help prevent blood from pooling, reduce swelling, help with healing, and reduce pain. With an ischemic ulcer: Do not wear compression stockings or wraps unless told to by your provider. These may restrict blood flow.
That’s why this isn’t a question to settle at home from a list. The same thing that treats one wound is something to avoid with the other unless a provider says so. Telling them apart depends on what is happening to the blood supply, which is found by examination.