1. Identify Your Skin Type: Before purchasing a blush stick online, it's crucial to understand your skin type. If you have oily skin, opt for a powder-based blush stick. For dry skin, a cream-based blush stick would be more suitable.

2. Understand Your Skin Tone: Your skin tone plays a significant role in choosing the right blush color. For fair skin, go for soft pinks and peaches. Medium skin tones can opt for rich mauves and deep peaches. Darker skin tones look great with deep fuchsias, berries, and warm browns.

3. Check the Ingredients: As a scientist, I can't stress enough the importance of understanding the ingredients in your cosmetics. Look for blush sticks that are free from harmful chemicals like parabens, sulfates, and phthalates. Opt for products with natural ingredients that are beneficial for your skin.

4. Read Reviews: Online reviews can provide valuable insights into the product's performance. Look for reviews that mention the product's texture, longevity, and pigmentation.

5. Brand Reputation: Purchase from brands that have a good reputation and are known for their quality products.

6. Return Policy: Since you're buying online, ensure the website has a good return policy in case the product doesn't meet your expectations.

7. Swatch Images: Look for swatch images online to get a better idea of how the color might look on your skin.

8. Check for Cruelty-Free and Vegan Labels: If you're conscious about animal welfare, look for brands that are cruelty-free and vegan.

9. Packaging: A blush stick with a twist-up mechanism is more convenient and hygienic than those you have to sharpen.

10. Price: Finally, consider your budget. While it's okay to splurge on a high-end blush stick, there are plenty of affordable options that deliver excellent results. Remember, expensive doesn't always mean better.

By keeping these tips in mind, you can make an informed decision when buying a blush stick online.