Is blood najis?

The evidence

The Quran mentions "blood spilled out" in a list of things that are forbidden and impure:

When a woman asked what to do about period blood on her clothes, the Prophet ﷺ told her to scrape it, rub it with water, wash it, and then pray in the garment (Sahih al-Bukhari 227). Many classical scholars reported agreement that flowing blood is impure. A minority of later scholars, such as ash-Shawkani, argued that only menstrual blood is clearly proven to be najis, while other blood is pure. Both views exist, but the majority view is the safer one to follow.

Small amounts are excused

Islam doesn't expect you to scrub every speck. All four schools excuse a small amount of blood, though they measure it differently:

• Hanafi: up to the size of a small coin (a dirham) is excused in prayer.
• Maliki: less than a dirham's size is forgiven.
• Shafi'i: small amounts, especially your own blood from pimples, scratches or bites, are excused.
• Hanbali: what people normally consider a small amount is excused.

Blood from mosquitoes and fleas is also forgiven. If you find a mark you're not sure about, you don't have to assume it's blood. See Can I pray with a stain on my clothes?

Blood that is pure

Not all blood counts as "spilled out". The blood left in the meat and veins of a properly slaughtered animal is pure, so a pink juice in your steak is no problem. Liver and spleen are halal to eat. The Hanafis also consider fish blood pure.

Is it the same as breaking wudu?

No, these are two separate questions. Whether blood is najis is about cleaning your body and clothes. Whether bleeding breaks wudu is a different debate: the Hanafis and Hanbalis say a lot of flowing blood does, while the Malikis and Shafi'is say it doesn't. See Does bleeding break wudu? Several Companions kept praying while wounded, which shows Islam's flexibility here.