Make the vinaigrette. In a small bowl, add all of the dressing ingredients and whisk well to combine. Set aside in the fridge to let the flavors come together.Build a medium-hot fire in a charcoal grill, or heat a gas grill to medium-high. (Alternatively, heat a cast-iron grill pan over medium-high heat.) Brush and oil the grill grates.Cut each romaine head in half lengthwise. Trim outside leaves but leave the stem attached; it holds the leaves together. Drizzle the cut side of each half with a little olive oil.Working in batches, if necessary, place romaine halves cut-side down on grill, and cook, turning once, until charred and slightly wilted, about 4 minutes.You want the lettuce to be a little wilted but still hold its shape.Transfer to a serving platter cut side up. Top with diced avocado and blueberries. Drizzle the vinaigrette over top and garnish with lemon wedges and chives.Serve immediately. You can either serve the hearts whole, or chop them and toss them for a salad.