Hb means hemoglobin in the medical context.
High Bloodpressure
Most often Hb stand for hemoglobin, which is a measure of the iron containing part of the red blood cell that carries oxygen.

In cardiology it can also stand for heart block.

Artists and draftsmen can use a variety of pencil leads in different hardnesses (e.g. H1, H2) and colors.

H=hard and B=black pencils are graded on how hard (therefore lighter line) or soft (therefore blacker line) they are an HB pencil is mid range between the 8H and the 8B.