Sure, here's the `database.py` file for the `app/api/db/` directory, which configures SQLite with `blog_app.db` using SQLAlchemy:

from sqlalchemy import create_engine
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import sessionmaker

SQLALCHEMY_DATABASE_URL = "sqlite:///blog_app.db"

engine = create_engine(
    SQLALCHEMY_DATABASE_URL, connect_args={"check_same_thread": False}
SessionLocal = sessionmaker(autocommit=False, autoflush=False, bind=engine)

Base = declarative_base()

def get_db():
    db = SessionLocal()
    try:
        yield db
    finally:
        db.close()

Here's a breakdown of the code:

1. We import the necessary modules from SQLAlchemy: `create_engine`, `declarative_base`, and `sessionmaker`.

2. We define the `SQLALCHEMY_DATABASE_URL` as a string with the SQLite database file path (`blog_app.db`).