Lear has just ordered Kent out of his sight, and Kent's defiant reply, "See better, Lear; and let me still remain / The true blank of thine eye," may be paraphrased, as "If it would help you to see better, Lear, I am willing to always remain here as the target of your angry looks." Also note that when Kent first objected to Lear's actions he addressed him as "Royal Lear"; now he addresses him simply as "Lear," an act of shocking disrespect.
Medieval Archery Target