I have a real hard calus on my big toe and it hurts almost to the point of tears. under the calus almost looks like a blood blister. is this serious?

Wart?: Sounds like you may have a wart. See your doctor.

Be seen: Not serious to the extent of likely being life threatening, but if you are in so much pain as actually being to the point of tears, I would get it looked at and treated. There are multiple potential possibilities, but most have pretty simple treatment options.

Get Checked Out: If it looks like a blood blister is forming would be good idea to be examined, many times a pressure ulcer can form beneath.

Not necessarily: It can be very painful. Apply emollient cream, wear soft shoes, soak in warm water and use pumice stone to scrub area. See your foot doctor or podiatrist

Pressure protection.: Having a callus on the toe is just not specific enough to understand your problem enough to offer a solution. The callous is usually on the bottom or medial side of the toe, but it can vary. It is typically caused by abnormal pressure applied over a period of time. The skin hardens to protect the underlying tissue. A foreign body or virus might also be considered. An examination would be best.