let serial1 = DispatchQueue(label: "com.besher.serial1")
let serial2 = DispatchQueue(label: "com.besher.serial2")

serial1.sync { // <---- we changed this to 'sync'
    for _ in 0..<5 { print("🔵") }
}
// we don't get here until first loop terminates
serial2.async {
    for _ in 0..<5 { print("🔴") }
}