Place poblano on a foil-lined baking sheet. Broil 8 minutes on each side or until blackened. Place pepper in a small zip-top plastic bag; seal. Let stand 10 minutes. Peel and chop.
Melt the butter in a Dutch oven over medium-high heat. Add onion and next 3 ingredients (through red pepper) to pan; sauté 4 minutes, stirring occasionally. Add corn; sauté 2 minutes. Remove 3/4 cup corn mixture from pan. Combine 3/4 cup corn mixture and 3/4 cup half-and-half in a blender; process until smooth. Add potato to pan; sauté 1 minute. Stir in 2 cups water; bring to a boil. Cook 4 minutes or until potato is almost tender. Reduce heat to medium.
Combine remaining 1/4 cup half-and-half and flour in a small bowl, stirring until smooth. Add flour mixture to pan. Cook 1 minute, stirring constantly. Return corn puree to pan. Stir in poblano, milk, and crab; bring to a simmer. Cook 3 minutes, stirring frequently.