docker-compose.yml

version: '2'
services:
  ghost:
    image: ghost
    ports:
    - 8090:2368/tcp
  blabla:
    image: rancher/lb-service-haproxy:v0.6.2
    ports:
    - 8091:8091/tcp
    labels:
      io.rancher.container.agent.role: environmentAdmin
      io.rancher.container.create_agent: 'true'

rancher-compose.yml

version: '2'
services:
  ghost:
    scale: 1
    start_on_create: true
  blabla:
    scale: 1
    start_on_create: true
    lb_config:
      certs: []
      port_rules:
      - hostname: ''
        priority: 1
        protocol: http
        service: ghost
        source_port: 8091
        target_port: 8091
    health_check:
      response_timeout: 2000
      healthy_threshold: 2
      port: 42
      unhealthy_threshold: 3
      interval: 2000

Without knowing or having the experience, but in your rancher-compose should it not be target_port: 8090 if target_port is necessary at all. In my opinion target_port is not neccessary as you link to the ghost service directly.

Additionally in your docker-compose-file why do you expose the ports with tcp? In my opinion that is not necessary it suffices the write

ports:
- 8091:8091