I have recently started exercising. I follow the Fitness blender videos to workout. In some of the videos, few mins of cardio followed by few mins of weights is shown. Is it wise to mix them both? The total duration is only 30 mins for the whole workout. Since I am a beginner am taking it slow.
What exactly do you mean by "wise"? It has its ups and downs depending on what your goals are.
If your goal is weight/fat loss then yes it is fine. You will burn through your glycogen stores via weights and most of the calories you burn via cardio will have to come from fat.
If your goal is strength, it's not the best. It's been shown that there is a linear interference between strength gains and cardio. Simply put, the more LOW INTENSITY cardio that you do, the more strength you will sacrifice (within reasonable limits). Hence why you will never see a powerlifter jog on a treadmill. If the cardio is high intensity, then I don't see a problem.
If your goal is to build muscle, then mixing in cardio will take away from the intensity of the workout, i.e you won't be able to give training your muscles 100% the intensity and concentration that you need. Alternative: do the cardio after your workout is done, or not at all, if your goal is muscle building (assuming your diet is proper).
If your goal is to just be "fit", then yes it's fine absolutely.
There is a problem with mixing them if you are going above 50-60% max. Your tendons get stretched during cardio. When you lift you want them tight. This can lead to numerous injuries but I would be most concerned about my knees. I am sure someone can give you a lengthy explanation but for the most part leave your cardio to after weights.
Some study has shown that if you do both heavy with few repetitions and lighter with higher repetitions, the muscles grow more than doing just one kind of workout. Also variations in sets can be used, like stripping weights lighter, then doing more reps.
You should start with warm up movements. That can contain various exercises and stretching. Some people warm up with lighter weights. You should do heavy compound weight lifting first, then other lifting, and any cardio later, because you can't do heavy lifting well as exhausted. It is also good practise to do heavy short 30 second sprints after workout, because it doesn't take long but activates the muscles well. Remember to stretch afterwards.
You could also consider the time between the sets. If it is shorter, there is less waiting around, but the weights get a bit lighter.