Blood in a cat's urine can occur with several diseases of the lower urinary tract. Idiopathic cystitis, urinary stones and, less often, bacterial infection are some possible causes. The symptoms often overlap, which means the cause can rarely be worked out just by looking at the cat or the urine.
Note whether your cat is peeing more often than usual, only passing small amounts, seems to be in pain or starts peeing outside the litter tray. The vet may need to feel the bladder and examine a urine sample. In some cases X-rays or ultrasound are used, for example to look for stones or other changes.
Blood in the urine should be assessed by a vet. If your cat is also straining without passing urine, especially if it is a male cat, the situation should be treated as an emergency. Lethargy, vomiting or obvious pain are further reasons to seek care quickly.