Hi Jenny, and welcome to the forum.
Mine began as ulcerative proctitis and is now either ulcerative colitis or Crohn's colitis (the latter being Crohn’s disease that is limited to the colon only).
Personally I have only ever suffered with diarrhoea out of the symptoms you listed, along with only minor weight loss. I agree with Cookie that it sounds more like Crohns disease. The rectum is normally involved in UC (it tends to begin there and then spread upwards), but anal issues and fissures tend to be more characteristic of Crohns, as does alternating diarrhoea and constipation. Most people with UC only suffer with D and it's usually bloody (which is the case with me).
Mouth ulcers may indicate Crohns too as the disease can affect you anywhere from mouth to anus; UC tends to be limited to the colon only, although extra intestinal symptoms such as mouth sores and joint pain are not unheard of.
Black blood would again indicate Crohns, as it would suggest you're bleeding higher up than the colon (i.e. in the small bowel) and partially digesting it, making it dark and tarry.
Dramatic weight loss is more common with Crohns (small bowel involvement) because it affects the are where you absorb nutrients from food, whereas the colon really just absorbs water. But it can occur with both forms of IBD.
Like Cookie said, it's different for everyone so it's impossible to say for sure. Hopefully your doctor can
Hope you get some answers and start feeling better soon!