Nobody can diagnose over the internet, but the characteristic meningitis rash, is a rash that appears underneath the skin, and the reason it doesn't blanch is that it's from blood leaking into the tissues under the skin. That's why it is so striking, and so deep in colour. It doesn't fade away because the blood is trapped in the tissues and has nowhere to go.
With viral rashes, the colour from the rash tends to be superficial, and pressure will divert the blood supply away from the area, so the area will blanch. When you release the glass, blood supply returns, so colour returns.
Please don't focus on her rash - if it was a true, non-blanching septicemic rash, you'd know and be in no doubt that you needed help. Focus on her overall condition, and trust yourself. You know your DD. If she didn't have the rash, would she seem very ill to you, or just a bit off-colour? Would you describe the rash as lacy? Where did it start? Has she had slapped cheek before?
DD3 is 8, and has just had slapped cheek. She had a full body rash, but the doctor said that it is a less usual, but legitimate, presentation of the condition. Her rash seemed to last an age, and left in the order that it appeared.