The Blue Card ([[§18g AufenthG|The paragraph of Germany's Residence Act that governs the EU Blue Card. It sets the degree-or-experience test and the salary thresholds]]) opens through one of two doors. You need only one.

Door 1 — a recognised university degree

You hold a university degree that is recognised in Germany (Part 3 covers how).
Your German job matches that qualification — a physics graduate hired as a physicist, not as a driver.

Door 2 — IT experience, no degree

You are an IT specialist with at least 3 years of comparable professional experience at academic level.
That experience must have been gained in the last 7 years.
No university degree is required. This is the door many Nigerian software engineers walk through.

Both doors also require a job offer or contract for at least 6 months, and a salary clearing the threshold in Part 2.

This is the sharp contrast with the Opportunity Card: that card lets you enter Germany with no job, to look for one over a year. The Blue Card assumes you have already secured the qualifying job. If you don't have one yet, the Opportunity Card may be your way in first — and the Blue Card the thing you convert to once you're hired.