Custom Field For a Content Type
Honestly, I'd recommend creating two separate fields, one for systolic pressure and another for diastolic pressure.  Then, use CSS to organize the fields the way you want.  It actually makes more sense to do it this way since they represent two distinct values, even though they're used together conventionally to represent blood pressure.

To expand on the answer, specifically on the CSS side, here are some additional tips.

Once you've found a way to isolate the two fields with classes, I recommend displaying both of their div wrappers as display: inline-block; and, for the first field (systolic pressure) give it a rule that uses the after pseudo-class to add the '/'.

.classes .isolating .systolic-field:after {
  content: "/";
}
.classes .isolating .systolic-field {
  display: inline-block;
}
.classes .isolating .diastolic-field {
  display: inline-block;
}


If you're not familiar with CSS I can provide a more "concrete" example, but this should be sufficient.