There are two common ways to control the crispness that you're looking for, and either might apply here.
Both approaches start the same way:
- Create a plane.
- Add two edge loops in each direction to create a rectangle in the center. To add an edge loop, hover the mouse near one of the edges and type CTRL r. Move the mouse to place the loop and left click. Right click to cancel.
- Extrude the rectangle in the Z direction.
This will create the basic shape. When you add a subdivision surface modifier you get something like this:
One approach is to use a bevel modifier or manual bevels along with the subdivision surface modifier. By controlling the bevel shape and width you can control the apparent curvature. Here is how to do it.
- Add a bevel modifier before the subdivision surface modifier.
- Adjust the amount and number of segments to taste.
Here's a sample Bevel Modifier setup:
Here's the resulting object
By making the bevel amount smaller you will make the edge transitions more crisp.
The other approach is to use edge loops as control loops along with the subdivision surface modifier.
Here is one way to do that:
- Add two edge loops to the extruded area and place them near the corners:
As you can see, the blob is starting to form up.
- Add two edge loops in the Y direction, "inside" the raised area and move them near the corners:
As you can see the blister is looking more like the packaging. You can move an edge loop by selecting it and tapping g twice. Right Mouse Button cancels the movement, while Left Mouse Button accepts the new position.