Blood type matters because if incorrectly matched blood is used in a transfusion, the recipient's immune system will attack cells in the donated blood. Severe cases can be fatal.
The patient's immune system reacts to antigens on the surface of blood cells. Antigens are proteins that immune cells recognize as not normally present in the recipient's body. Immune system cells produce antibodies, substances that target the antigens and the cells to which they are attached.
The purpose of blood typing is to prevent the introduction of unfamiliar antigens into patients' blood. "Blood type" is a description of which antigens are present in a person's blood, specifically the ABO blood group and the Rh factor.
All humans fit into one of four ABO blood groups. If their blood contains the A antigen only, they are Type A. If it contains the B antigen only, they are Type B. If it contains both, they are Type AB. If neither antigen is present, they are Type O.
Type A transfusion recipients must avoid getting the B antigen. They can be given Type A or Type O blood.
Type B transfusion recipients must avoid getting the A antigen. They can be given Type B or Type O blood.
Type AB transfusion recipients already have both A and B antigens. They can be given A, B, AB, or O blood.
Type O transfusion recipients lack both A and B antibodies, and must avoid receiving either one. They can be given only Type O blood.
The Rh factor is a different antigen. It is simpler than ABO blood groups in that people either have it or they don't. Individuals who have it are "Rh positive" and those who do not are "Rh negative." According to the same principle of avoiding giving blood that contains an antigen not present in the patient's body, Rh negative patients must not be given Rh positive blood. Rh negative blood can be given to both Rh positive and Rh negative patients.
The discovery of Rh factor is an interesting story, and I've included a link. Antibodies to Rh factor can cross the placenta and attack the blood of a fetus, so Rh incompatibility between an Rh negative mother and her Rh positive fetus once led to miscarriages, stillbirths, and neonatal death. Today a shot of Rh immune globulin can be given to prevent expectant mothers from developing Rh antibodies.
In answer to the question, blood that lacks any of the surface antigens described could be given to anyone without triggering an immune response because of ABO or Rh incompatibility. The type that lacks any of these antigens is Type O negative, or O-.
Because other antigens are present in blood, normally blood is "cross-matched" to check for the presence of antibodies that could cause a problem by mixing a small amount of donor and patient blood and looking for agglutination (clumping together of red cells) under a microscope.
https://emedicine.medscape.com/article/797150-overview?pa=wGChXLMBCG3rJizgeJDCS2FW5%2FlSh9sOh%2BY2vxxqFR6WDbi8gWMyJqO2otI6LtfkDzum8bfU2trpB%2B8OdCa%2BrG8OxiDO6aXcc0JeYePWHqw%3D
https://www.ncbi.nlm.nih.gov/books/NBK2265/
The blood type O negative is considered the universal donor type. There are four blood types: A, B, AB, and O. Additionally, blood is categorized according to the presence or absence of a red blood cell antigen known as the rhesus factor. The presence of that factor is RH positive and absence is RH negative. When a blood transfusion is needed, it is vital that a compatible donor type be introduced into the body of the recipient in order to avoid detrimental interactions. Ideally, the donor's blood type will exactly match that of the recipient. In an emergency situation, when a matching blood type is unavailable, blood type O negative can be used to provide transfusions to those with other blood types. The term “universal donor” refers simply to the type and not to other factors that can impact the recipient of a transfusion, however. Even blood type O negative can contain antibodies that result in negative interactions in transfusions of recipients with other blood types.
https://ashpublications.org/blood/article/5/1/61/6922/DANGEROUS-UNIVERSAL-DONORS-I-OBSERVATIONS-ON
https://health.ucdavis.edu/transplant/livingdonation/donor_compatible.html
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